Outside a theatre in Shaftesbury Avenue that offers a dubiously amusing entertainment a poster proclaims: ‘Pant-wettingly funny.’ This is interesting, because what one might have the misfortune to wet is not a pant but pants. The grammar, though, is undoubtedly correct. Nouns used as adjectives generally remain in the singular. This rule makes honest nouns with a singular meaning, but a plural form, shrink into singularity once they are deployed adjectivally: trouser-pocket, not trousers pocket.
It is not as a simple as that, naturally. Take the rather silly term drug czar (or drug tsar). When in 1982 the United States appointed someone in charge of its policy on drugs, the news agency UPI announced him as the new drug czar. (The word czar had been used a decade earlier with reference to the man in charge of energy.) The Oxford English Dictionary records drug tsar in a citation from 1989. But Keith Hellawell, who was Britain’s equivalent from 1998 to 2002, was often referred to as a drugs czar.
If justification is sought for the plural form in drugs czar, people will sometimes say that it is not any old drug (of the kind made by drug companies) that is under consideration, but drugs, as in ‘sex and drugs and rock-and-roll’. Certainly there is a distinction between, ‘He had to take many drugs’, and, ‘He was out of his mind on drugs’. But the distinction does not apply to drug addict or drug-pusher at the corner of the street. Both drug baron and drugs baron are used.
However there are parallels for drugs czar. A drink party is one where (strong) drink is served; but it is often called a drinks party. When the sun is over the yard-arm it is for some drink time, for others drinks time.

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