Etymology
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ineffectual (adj.)
early 15c., from in- (1) "not, opposite of" + effectual. Related: Ineffectually; ineffectuality.
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half-assed (adj.)
"ineffectual," 1932; "Dictionary of American Slang" suggests it is perhaps a humorous mispronunciation of haphazard. Compare half-hearted.
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unavailing (adj.)
"ineffectual, useless," 1660s, from un- (1) "not" + availing. Also see unavailable.
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dumb-ass (n.)

also dumbass, "stupid, ineffectual person," by 1959, from dumb (adj.) "stupid" + ass (n.2).

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numbnuts (n.)
stupid or ineffectual person, by 1971, U.S. slang, from numb (adj.) + nuts "testicles;" with suggestion of impotence.
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stuffed (adj.)

mid-15c., in reference to garments, "padded with stuffing," past-participle adjective from stuff (v.). Hence stuffed shirt "pompous, ineffectual person" (1913).

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non-starter (n.)

also nonstarter, "one who does not start a race, contest, etc.," hence "ineffectual person or impracticable idea," 1909, from non-  + starter.

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unavailable (adj.)
1540s, "ineffectual," from un- (1) "not" + available. Meaning "incapable of being used" is recorded from 1855. Unavailing has taken up the older sense of the word.
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twit (n.)
"foolish, stupid and ineffectual person," 1934, British slang, popular 1950s-60s, crossed over to U.S. with British sitcoms. It probably developed from twit (v.) in the sense of "reproach," but it may be influenced by nitwit.
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