"Jagoff" (sometimes "jag-off") seems to have originated in Pittsburgh but is also recognized in the Midwest as slang for an inept, feckless, contemptible, or generally worthless person, a
loser, a "schlemiel."
The term is almost certainly derived from the
verb "jack off" (through noun "
jack-off") as in "masturbate," but somewhat
like the British use of "wanker," it is usually not a direct comment on self-pleasuring, but more of a general term of contempt or deliberate abuse.
Like "wanker," "jagoff" is somewhat vulgar and not to be used lightly, and avoided in cultivated speech, but is recognized by all in the regions in which it has currency.
.
The book KILLER
CLOWN relates that John Wayne Gacy became especially flustered or
angry when called a "jagoff." So the police deliberately used that term to
throw him off-balance during interrogation.
.