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in the same vein as "go forth and multiply", "piss off", "fuck off" and so on...

origin: warrington, england
...will you please knob off, dickhead...
by critch April 13, 2005
To masturbate, fap, jack off, wank, me-time, choke the chicken, spank the monkey, beat the meat, squeeze the cream, charming the snake, manual stimulation of the penis.
"Gonna go knob off for a bit", said Jeff.
by FilthyHobo420 June 12, 2012
a particularly un-arousing experience

a floppy cock

an un-erect penis

opposite of knob on

U.k. (slang) Tell someone to go away
seeing my brother with my moms dildo gave me such a knob off

I have such a big-ass knob off right now

Knob off you little twat
by Tom P. Cock March 31, 2006