plank, n.¹ — Green’s Dictionary of Slang

plank n.¹

[? plank v. (1) or the SE planks beneath which it is hidden, or the wooden box in which it is kept]

(Irish) a cache of money.

1958		(con. 1950) E. Frankel Band of Brothers 62: You’re off your rocker. You’ll give them the idea you got a plank.
1996		(con. 1950s) C. Kenneally Maura’s Boy 20: My mother had a ‘plank’ there, a secret stash of cash in a yellow teapot.