There are three groups of (plural of compound nouns)
1. Old Compounds.
Our oldest compounds and most of our newer ones are forms representing a unit of thought, hence are treated as simple nouns, the final element, if a noun, assuming the plural form that it would have as a simple word: toothpick, toothpicks; horseman, horsemen; housetrap, housetraps; woman hater, womanhaters; washerwoman, washerwomen; bird's-nest, bird's-nests or birds' nests; crow's-foot, the crow's-feet about his eyes; Peter's penny, Peter's pence; etc. If the final element is not a noun, the plural usually ends in -s: bucketfuls, handfuls, spoonfuls, breakdowns, drawbacks, setbacks, hold-ups, stop-overs, stowaways, godsends, merry-go-rounds, go-betweens, forget-me-nots, cure-alls, four per cents (last element a phrase), four o'clocks, etc.Inflection usually takes place only in the final element, but in certain compounds containing two nouns as components both nouns have plural form: maid servants, girl cashiers, woman (more commonly women) clerks, chief justices, lieutenant colonels, lieu tenant governors, etc., but men servants, men friends, gentlemen boarders, women students, women singers, etc. It would be more in accordance with our modern feeling, to construe the first element in many such expressions as an adjective, and the following word as the governing noun. Some of these expressions, however, such as chief justice, lieutenant governor, etc., are real compounds.
In a few words a difference of accent is associated with a differ ence of meaning: mankind (the human species), mánkind (the males of a household, the male sex) or sometimes still in accordance with older usage ménkind. In America the colloquial expression is men folks. Womankind has the force of all women, while woman kind or wómenkind means women folks ('our, his, etc., women folks'). In England the forms menfolk and womenfolk are used instead of men folks and women folks. Americans often employ menfolk, womenfolk in literary style: 'As the wife of a celebrated senator, mother of his successor in the Senate and of the Governor of Wisconsin, her (i.e. Belle Case LaFollette's) personality along with her work was merged in the fame of her menfolk' (The New York Times, Editorial, Aug. 20, 1931). Similarly in formal language we say 'my (his, etc.) kinsfolk,' but colloquially we use more com monly 'my (his, etc.) relatives,' 'my (his, etc.) people,' or 'my (his, etc.) folks.' 'My (his, etc.) folks,' however, refers most commonly to the members of a single household.
2. Syntactical Compounds.
Although in the old compounds in 1 the components stand in a syntactical relation, the relation is often not indicated by their form; hence we can easily add the plural sign at the end of the group. But in many compounds where the syntactical relation is indicated by the form, especially where the first component is a noun which is modified by a genitive, prepositional phrase, adverb, or adjective, we plainly feel the force of the noun and give it plural form if there is a reference to more than one: men-of-war, mothers-in-law, brothers-in-law, sons-in-law, daughters-in-law, heirs-at-law, commanders-in-chief, editors-in-chief, aides-de-camp, autos-da-fé, lookers-on, goings-on or sometimes on-goings (both usually in the plural), passers-by, coats of-mail, justices-of-the-peace, postmasters-general, governors-general, attorneys-general, courts-martial, notaries public, poets laureate, knights-errant, billets-doux. In a number of such compounds, however, the concrete force of the noun is felt so little and the oneness of the compound is felt so strongly that the regular plural ending -s is added at the end: will-o'-the-wisps, good-for-nothings, jack-in-the-pulpits (American plant), brigadier generals, major generals, lieutenant generals, attorney-generals (perhaps more com mon than attorneys-general), etc.; often court-martials instead of courts-martial; in popular speech often mother-in-laws, etc. While we say in the literary language 'my brothers-in-law' we must, on the other hand, say 'my brother-in-law's house."When in compounds the second component is a noun in apposition with the preceding component, both components usually have plural form: Knights Templars, Knights Hospitalers, Lords Justices, Lords Lieutenants, Lords Chancellors, etc. But there is considerable fluctuation in usage with some of these words: Lord Lieutenants (both components felt as a unit), Lords Lieutenant (second component felt as an adjective), etc.
