Green’s Dictionary of Slang

pronk n.

[? prat n.1 (5) + ponce n. (2) + wank n. (1)]

a fool, an idiot.

[US] in W.C. Fields By Himself (1974) 245: He’s a pronk wonk.
[UK]C. MacInnes Absolute Beginners 25: Here the pronk half rose in his ballet tights and saluted.