Learn English – Meaning of once and while in “once in a while”

Strange as it seems, cannon does appear to have once been a mass noun, like rain or infantry. Instead of saying rains, one says drops of rain. Similarly, instead of saying cannons, it appears that one either said cannon or pieces of cannon. Consider the google Ngram below:

Here, the curve for two cannon is higher than it should be, because of constructions like two cannon balls.

Tsuyoshi is right about cannons being the plural in the 1500's; Google Ngrams doesn't have adequate data before early the 1700's, but we can check Shakespeare, who uses cannon as a regular count noun. So the plural has gone from cannons, to pieces of cannon, to cannon, and back to cannons.

What's happened is that the verb to sanction has retained the original sense relating to endorsement/recognition by official decree.

But the noun sanctions (invariably pluralised, frequently imposed or applied) has come to mean measures taken by authority (often, multiple cooperating authorities) to discourage unsanctioned activities. By default, it's usually governments restricting trade in certain goods and services with some other nation, in order to put pressure on its government.

There is also the "positive" singular noun give (ones) sanction, but as that link shows, it's rapidly declined as the "negative" plural noun impose sanctions has gained currency.

Although superficially this looks odd, in practice there's unlikely to be any confusion because the verb / plural noun distinction is almost always made.

Having said all that, usage does change. Increasing numbers of younger speakers rarely hear the verb or singular noun form with positive associations, and they effectively "back-form" a new verb form they want to mean to impose sanctions. @Jay has identified a few such usages already in "print", and much as they make me cringe, doubtless there will be more in future.