a. Full of: playful.
b. Characterized by; resembling: masterful.
c. Tending, given, or able to: useful.
2. A quantity that fills: armful.
[Middle English, from Old English, from full, full; see full1.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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.
[Old English -ful, -full, from full1]
Usage: 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
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
a suffix meaning “full of,” “characterized by” (beautiful; careful); “tending to,” “able to” (harmful; wakeful); “as much as will fill” (spoonful).
[Middle English, Old English -full, -ful, representing full, ful full1]
usage: The plurals of nouns ending in -ful are usu. formed by adding -s to the suffix: two cupfuls. 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.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.