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.    1