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Plurals of compound nouns
The majority of English compound nouns have one basic term, or head, with which they end, and are pluralized in typical fashion:
able seaman
able seamen
head banger
head bangers
yellow-dog contract
yellow-dog contracts

A compound that has one head, with which it begins, usually pluralizes its head:
attorney general
attorneys general
bill of attainder
bills of attainder
court martial
courts martial
fee simple absolute
fees simple absolute
governor-general
governors-general
passerby
passersby
ship of the line
ships of the line
son-in-law
sons-in-law
minister-president
ministers-president
knight-errant
knights-errant
procurator fiscal (in Scotland)
procurators fiscal

It is common in informal speech to instead pluralize the last word in the manner typical of most English nouns, but in edited prose, the forms given above are preferred.
If a compound can be thought to have two heads, both of them tend to be pluralized when the first head has an irregular plural form:
man-child
men-children
manservant
menservants
woman doctor
women doctors

Two-headed compounds in which the first head has a standard plural form, however, tend to pluralize only the final head:
city-state
city-states
nurse-practitioner
nurse-practitioners
scholar-poet
scholar-poets

In military usage, the term general, as part of an officer's title, is etymologically an adjective, but it has been adopted as a noun and thus a head, so compound titles employing it are pluralized at the end:
brigadier general
brigadier generals
major general
major generals

For compounds of three or more words that have a head (or a term functioning as a head) with an irregular plural form, only that term is pluralized:

man-about-town
men-about-town
man-of-war
men-of-war
woman of the street
women of the street

http://en.wikipedia.org/wiki/English_plural