A compound word is made up of two or more nouns.
Example: tooth + brush = toothbrush
Compound words may be divided into two classes:—
(1) Those whose parts are so closely joined as to constitute one word. These make the last part plural.
Example: courtyard, dormouse, Englishman, fellow-servant, fisherman, Frenchman, forget-me-not, goosequill, handful, mouthful, cupful, maidservant, pianoforte, stepson, spoonful, titmouse
(2) Those groups in which the first part is the principal one, followed by a word or phrase making a modifier. The chief member adds -s in the plural.
Example: aid-de-camp, attorney at law, billet-doux, commander in chief, court-martial, cousin-german, father-in-law, knight-errant, hanger-on.
NOTE.—Some words ending in -man are not compounds of the English word man, but add -s; such as talisman, firman, Brahman, German, Norman, Mussulman, Ottoman.