a suffix meaning “full of,” “characterized by” (shameful; beautiful; careful; thoughtful); “tending to,” “able to” (wakeful; harmful); “as much as will fill” (spoonful).
Usage note: The plurals of nouns ending in -ful are usually formed by adding -s to the suffix: two cupfuls; two scant teaspoonfuls. Perhaps influenced by the phrase in which a noun is followed by the adjective full (both arms full of packages), some speakers and writers pluralize such nouns by adding -s before the suffix: two cupsful.
