A person of my acquaintance, who shall remain anonymous, recently told me that she uses three spoonsful of sugar in her coffee. There went that "fingernails on the blackboard" feeling again. Be proud of me, as I refrained from correcting her on the spot. Hopefully, she will read this and learn the error of her ways.
A compound noun is a noun that is formed from two or more words: a principal word and another word or words which modify it. When forming the plural of a compound noun, it is customary to pluralize the principal noun, rather than the entire word. Examples of compound nouns and their plurals:
- Board of Education/Boards of Education
- Attorney General/Attorneys General
- Chief of Staff/Chiefs of Staff
Through common [mis]use, many of the above are becoming acceptable in their technically incorrect forms (e.g. court-martials, cul-de-sacs, and attorney generals). Dictionaries often list both forms. This is not true of spoonsful, which is not now, and never has been, a word.
Following the logic above, it would seem that my friend was right when she said spoonsful; however, this is English, and logic does not apply.
Since spoonful is not truly compound noun (ful is not a word, thus it cannot form a compound) the rule is:
When a noun is in the form [container]ful (e.g.bucketful, cupful and handful), an s is added to the end to form the plural (spoonfuls, cupfuls and handfuls).