American 'Diner Lingo' seems to consist largely of humorous crossword-style references (Noah's boy = Slice of Ham, Mother and child reunion = chicken and egg sandwich, Dog soup = water, etc).
Most seem fairly obvious in their origin but I'm totally at a loss to work out why a "mystery in the alley" is equivalent to "a side order of hash".
Is it simply that it looks like a big mess (e.g. that's been swept up from the alley behind the diner) or is there a more subtle derivation that I'm overlooking?
Best Answer
J.S. Farmer & W.E. Henley, Slang and Its Analogues, volume 4 (1896) has a suggestively similar entry for mystery:
Eric Partridge, Dictionary of Slang and Unconventional English, fifth edition (1961) has this:
The use of "bag of mystery" appears to go back at least as far as 1873, where it is used as a synonym for the French harlequin and the English Punch's Puzzle. From an 1873 translation of Eugene Sue, The Mysteries of Paris [combined snippets]:
A footnote linked to the phrase "bag of mystery" offers this explanation:
An 1845 translation of Sue’s novel, however, refers to the “bag of mystery” as a harlequin, and then observes in a footnote,
J.E. Lighter, Random House Dictionary of American Slang, volume 2 (1997) has this entry for mystery:
Lighter then provides a list of 24 citations ranging from 1877–78 to 1996 and indicating a fairly impressive continuity of usage for mystery and mystery meat over those years. The first mention of hash in connection with mystery is the first citation:
Other instances come from 1888, 1906, and 1921:
"Mystery meat" dominates the later references—those from the 1960s onward—in Lighter.
"Mystery in the alley" probably combines the old idea of "mystery [meat]" either with the old commonplace description of a thing of dubious provenance as having been found "in the alley" or with (as Peter Shor suggests in his answer) a separately derived (and diner-specific) notion of "in the alley" as code for "on the side."