The suffix -ful comes from the Old English adjective full, meaning “full.” Full was commonly added to a noun in order to form adjectives meaning “full of, characterized by” whatever quality was denoted by the noun: playful, careful. The use of -ful to form nouns meaning “a quantity that would fill” a particular receptacle (cupful, mouthful) also goes back to Old English. In modern usage the correct way to form the plural of these nouns is to add an s to the end of the suffix: cupfuls.
