Green’s Dictionary of Slang

(US) fellatio.

1998		The LOX ‘Money, Power & Respect’ 🎵 (Jan. 13) ‘Get This $’: Hotter in the club getting brain from a diva.
2004		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.
2004		Jin ‘Club Song’ 🎵 I wasn’t talking about your IQ when I said ‘Give me some brains’.
2015		http://www.xvideos.com 🌐 Natural busty japanese giving brain - 5 min.