Green’s Dictionary of Slang

1. virile, having a large penis.

1654		Mercurius Fumigosus 54 22-30 Nov. 4: The good Women[...] began to talke of the sufficiency of their Husbands in the Art of generation; Truly saith one, I thank God my Husband is as timbersom a man as can ly by a womans side, and as willing to do Duty as any man can be; Aye quoth another Gossip, my Husbands timbersome enough too, and hath a Trap-stick more like a Constables-staff then a watch bill, yet will do no duty at all, though I have taken it in my hand, and gave it the word of command, bid it stand, nay come before the Constable, and yet the feeble water-ratt will hang his head, and never so much as know me.
1728		T. Walker Quaker’s Opera 31: Coax. Why I was timbersome, my Love, I was timbersome. Wife. Timbersome! ay, you’re always so.
1749		Fielding Tom Jones in Works I (1882) 413: [of Jones’ father] [H]e is a timbersome man every body knows.

2. keen on, enthusiastic.

1871

H. Holt Golden Bait III 229: Oh , you’re a slippy chap, a shallywalley feller , an’ as timbersome o’ doing yoursel a service as a mealy-mouthed lass o’ saying ‘yes’.