plank, n.³ — Green’s Dictionary of Slang

plank n.³

the penis; usu. in combs. below.

plankspanker (n.) [spank v.¹ (4); plus pun on plank-spanker under plank n.²]

a masturbator.

2000

M. Manning Get Your Cock Out 93: Mincey forgot about the planned humiliation of the prodigal plankspanker.

yank the plank (v.)

see under yank v.¹