Green’s Dictionary of Slang

brain n.2

also brains
[play on head n. (2c)]

(US) fellatio.

[US]The LOX ‘Money, Power & Respect’ [lyrics] (Jan. 13) ‘Get This $’: Hotter in the club getting brain from a diva.
[UK]Guardian 6 Nov. 3: The Metropolitan Transit Authority vowed yesterday to start removing the adverts after being informed that the slogan plastered over 200 city buses – ‘Read books, get brain’ – refers to a slang term for oral sex.
[UK]Jin ‘Club Song’ [lyrics] I wasn’t talking about your IQ when I said ‘Give me some brains’. [Internet] Natural busty japanese giving brain - 5 min.