Learn English – Why does “keep tabs on” mean what it means

Christine Ammer, The American Heritage Dictionary of Idioms (1997) has this:

keep tabs on Observe carefully, keep record of. [Example omitted.] This expression uses tab in the sense of "an account." {Late 1800s}

The Wordsworth Dictionary of Idioms (1982) concurs:

keep tabs on (someone or something) to keep a check on (someone or something); to watch (someone or something): [example omitted]. {Originally US, from an American use of tab to mean 'account'.}

So "keep tabs on" originally meant simply "keep accounts of."

Early newspaper occurrences of 'keep tab[s]'

The earliest instances of "keep tab[s]" in the Library of Congress's newspaper database are from the period 1883–1885. In this early phase of its use, the idiom seems to have more frequently involved the singular tab and the preposition of or for than the plural tabs and the preposition on.

From "The New Broom," in the St. Paul [Minnesota] Daily Globe (June 7, 1883):

"Are there any other games going on tonight?"

"Not keeping tabs for the games I can't say. But you mark my words, this and other dispensations he contemplates introducing under the name of reform, will fall by their own weight."

From "The City," in the Las Vegas [New Mexico] Daily Gazette (March 27, 1884):

The evening paper states that the concert to be given by Prof. Boffa and his orchestra "has been postponed to Wednesday, April 2," the date fixed upon. Nothing like keeping tab and laying for sleepers.

From "Coast to Coast: Were the Excursionists from California Passing Through Las Vegas Yesterday" in the Las Vegas [New Mexico] Daily Gazette (May 10, 1884):

"You don't mean to say that all these happy go lucky looking gentlemen sauntering about the platform are ecclesiastical dudes, do you?" queried the reporter.

"Not by considerable," replied the scribe's newly found friend. "You understand the male excursionists are not all ministers, but taking advantage of the low rates; people came from all sections of the state, and altogether there are about one hundred and fifty people on this train."

"Does Mr. Shearer [the church organizer] keep tab for the excursionists and pour out salvation at every stopping point?"

From "The Convention: Who Will Be the Helmsman of the Republican Ship of State Still Undecided," in The [Ossowo, Michigan] Times (June 6, 1884):

In point of noisy enthusiasm the [James G.] Blaine men seemed to have decidedly the best of it [over the Chester A. Arthur men]. ... But it was generally observed that no conversions were the apparent result of this exchange of heated opinions, and that the Blaine men got it as hot as they gave it to the supporters of Arthur. ... The managers of the Blaine boom claimed, however, to have made great progress. Mr. Blake, of the New York Tribune, who keeps "tab" of the Blaine vote, claimed last night 333 votes on the first ballot, and promises of more from delegates who had before been considered solid for Arthur.

From "Vicinity Items," in the Ottawa [Illinois] Free Trader (June 13, 1885):

Senator Logan's son is not particularly popular in Springfield. He is regarded as a cross between a dude and a lazy boy.—His father's young secretary, Mr. Albert Hall—who, by the way, is a bright, energetic fellow—seems to share the general dislike of Logan, the son. At a party in Springfield the other evening young Logan was asked by a lady how long he had been in Springfield. Turning listlessly to Hall, he inquired in drawling tones, "Al-bert, old fel, how long hve I been in Springfield, eh?" Mr. Hall retorted rather sharply: "Excuse me, sir, but I'm not keeping tabs for the benefit of your feeble memory."

From "Street Sayings," in the St. Paul [Minnesota] Daily Globe (November 18, 1885):

Senator A.E. Rice of Willmer was in the city yesterday. He was offering to bet hats—good seven and three-quarter Dunlaps—that there will be no extra session of the legislature this winter. When last seen he has a good string and was keeping tab that none might get away.

From "Spirit of the State Press," in the [Indianapolis] Indiana State Sentinel (November 25, 1885):

Secretary Manning has ordered the Doorkeeper of the Treasury Department to "keep tab" on the employes and see what time each one gets to work in the morning. A good many of them are in the habit of ambling leisurely around at 10 o'clock an after, and the Secretary is bound to enforce a little more promptitude.

And from "Fan Fancies in Pencil," in the St. Paul [Minnesota] Daily Globe (December 20, 1885):

No one ever forgot an engagement with this young lady. She is rich and can afford to wear poor clothes. That is why the young men always remember. She is a blond, belongs to the church, but wears decollotte dresses and so has need for a fan for purely dress purposes. But she carries one, nevertheless, and uses the sticks to keep tab of the compliments paid her during the evening. She has just recorded the lucky seventh and is looking about for the next victim.

In the period 1886–1888, the idiom became increasingly popular and seemed to standardize on the form "keep tab on." But not until 1889 did the exact plural wording "keep tabs on" appear in the Library of Congress database. From "Phases of Life," in the St. Paul [Minnesota] Daily Globe (July 8, 1889):

Did you ever notice that all the safest, darkest, coolest and most alluring spots of shade in the park are on the flats hedged in with "Keep off the grass?" If you think it is a fancy just ask a park policeman about some hot afternoon when the water in the lake is cooking the ducks. An old fellow who keeps tabs on the visitors at Central park spoke of it yesterday.

Conclusions

One striking aspect of the rise of "keep tab[s] on" is how many of the early results come from one newspaper, the St. Paul Daily Globe. That newspaper registered the first recorded form of the phrase ("keeping tabs for"), in 1883; among the first instances of the early standard singular form of the idiom ("keep tab on"), in 1886; and the first occurrence of the plural form that would eventually become standard ("keep tabs on"), in 1889—an instance that may have been a typo. The first newspaper to use the wording "keep tabs on" more than once was actually the Helena [Montana] Independent, in April and July 1890.

The mystery is why the shift from singular tab to plural tabs occurred. It is somewhat as if "keep track of" suddenly became "keep tracks of." But given that the earlier "keep tab on" was still quite a novelty in most of the United Sates, its further evolution is not astonishing.