-ful

1.
a suffix meaning “full of,” “characterized by” (shameful; beautiful; careful; thoughtful); “tending to,” “able to” (wakeful; harmful); “as much as will fill” (spoonful).
Origin
Middle English, Old English -full, -ful, representing full, ful full1
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.
British Dictionary definitions for -ful

-ful

suffix
1.
(forming adjectives) full of or characterized by: painful, spiteful, restful
2.
(forming adjectives) able or tending to: helpful, useful
3.
(forming nouns) indicating as much as will fill the thing specified: mouthful, spoonful
Usage note
Where the amount held by a spoon, etc, is used as a rough unit of measurement, the correct form is spoonful, etc: take a spoonful of this medicine every day. Spoon full is used in a sentence such as he held out a spoon full of dark liquid, where full of describes the spoon. A plural form such as spoonfuls is preferred by many speakers and writers to spoonsful
Word Origin
Old English -ful, -full, from full1
Word Origin and History for -ful

word-forming element meaning "full of, characterized by," Old English -full, -ful, suffix use of full (adj.).