Some speakers have trouble with nouns that end in -ful, puzzling, for example, between cupfuls and cupsful. This is a case in which common sense should probably advise against consulting M-W.
Although the M-W entries for cupful, handful, and armful list the plurals cupfuls, handfuls, and armfuls first, they give cupsful, handsful, and armsful as alternative spellings. In addition, the spelling handfull is in there as an “also.” My American spellchecker does not countenance any of these alternatives.
Cupsful doesn’t cut it because compound nouns are made up of two or more words that can be used on their own. For example, the words in the compound policeman can be used separately: “The man called for the police.” The element ful in cupful is not a word; it’s a suffix.
Common sense tells me that cupsful is incorrect.
