There are cases when just adding –s to form the plural of a noun doesn’t work. That’s because the noun is a compound noun formed by two or more words. A classic example is brother-in-law. Where will the –s should be added to form the plural?
First of all we have to look at the three different types of compound nouns. They are: open, closed, and hyphenated.
The open compound nouns are written as two or more separated words, such as the word "post office". Closed compound nouns are nouns that combine two words, such as the word "sunrise" (made from sun + rise). The hyphenated compound nouns are nouns made from two or more words separated by hyphens, such as the word given as example in the first paragraph, "brother-in-law".
To form the plural of an open compound noun you have to add –s to the principal word of the compound (example: post offices).
To form the plural of a closed compound noun you just have to add –s at the end of the word (example: sunrises).
To form the plural of a hyphenated compound noun you have to add –s to the principal word of the compound (example: brothers-in-law).
Most of the dictionaries (such as Dictionary.com) list the plural of armful as being armfuls; however, some dictionaries (such as Merriam Webster) give a second option as being armsful.