numb handn.colloquial (now English regional) a clumsy or inexpert person.
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the world > action or operation > ability > inability > unskilfulness > [noun] > clumsiness or awkwardness > person
numb hand1850
slummocker1880
1850 R. S. SurteesSoapey Sponge's Sporting Tour xxxviii, in New Monthly Mag. Mar. 376 The fact is, that ladies'-maids are only numb hands in all that relates to hunting.
1868 J. C. AtkinsonGloss. Cleveland Dial. 359 Nobbut a numb hand.
1928 A. E. PeaseDict. Dial. N. Riding Yorks. 88/2 Nobbud a noom hand at t'job.