Green’s Dictionary of Slang

spur n.

the penis.

[Ire]Purgatorium Hibernicum 27: Until her rider Curbd the bride- / Delt and clapt Spurrs into her side / And prickt her up .
W. Dunkin Parson’s Revels (2010) 62: For Spurs [...] / Were made for Birds of mettle Keen, / [...] / Embassadors, politely bred, / Without them never go to bed / To guaranty a maidenhead.
[UK]‘John and the Maid’ in Rumcodger’s Coll. in Spedding & Watt (eds) Bawdy Songbooks (2011) IV 265: He now grew tir’d, and wish’d much to dismount, / His ass it stood still, for his spur it was blunt.

SE in slang uses

In phrases

have a spur in one’s head (v.)

of a fellow jockey, to be an honest person, to be no coward.

[UK]Gent.’s Mag. XL 560: To express the Condition of an Honest Fellow and no Flincher, under the Effects of good Fellowship, he is said to [have] Got a spur in his head; this is said by brother jockies of each other.
Courier (Asheboro, NC) 13 Aug. 6/1: Mr Jones was a tall, slender wiry man. He was a man who had a spur in his head, wide awake, enterprising and a rusher.