Green’s Dictionary of Slang

(US) a minor scar or bruise, an acne spot etc; often used to a child who has suffered such a wound.

1964		H. Selby Jr Last Exit to Brooklyn 31: Whats the matta Georgie? Has the poor girl got a Booboo?
1967–8		Baker et al. CUSS 85: Boo-boo A pimple.
2000		F. Kellerman Stalker (2001) 308: Dad, I think I’m beyond kissing the boo-boo.
2013		T. Robinson Hard Bounce [ebook] ‘[D]o you wanna help me move this fucking thing or should I kiss your boo-boo first?’.