Green’s Dictionary of Slang

1. (US) the penis [pun on wood n.¹ (6)].

1972		B. Rodgers Queens’ Vernacular 211: large cock [...] lumber.
1973		(con. 1940s–60s) Hogbotel & ffuckes ‘The Ballad of Professor John Glaister’ in Snatches and Lays 84: If a fossilized foetus you find up your lumb, / Or your wife shoves a poker red-hot up your bum.
1981		National Lampoon June 54: When I finally slipped the lumber to her, we both began screaming [HDAS].
a.1990		C.T. Westcott Half a Klick 122: Fucker...loafed his lumber to some woman [HDAS].

2. (US drugs) unwanted twiggy stems in marijuana.

1974

in National Lampoon (Jan. 1975) 43: It was ninety bucks a metric lid, but at least it doesn’t have any ‘lumber’ [HDAS].