Green’s Dictionary of Slang

1. a nonsensical idea.

1984		LaBarge & Holt Sweetwater Gunslinger 201 (1990) 80: ‘I got a better idea,’ said Slim. ‘Oh, oh. Another brain fart,’ snickered Sundance.
1993		Eble Campus Sl. Apr.
2004		posting at depthmarker.blogspot.com 3 Jan. 🌐 I volunteered...out of a charitable moment (the bartender clarified charitable as a brain fart!).
2016		Guardian CiF 10 Dec. 🌐 Bet you wish you could delete that little brain fart.

2. a loss of memory or concentration.

1989		P. Munro Sl. U. 44: In the middle of the sentence he blanked out — ‘. . . which leads directly to my . . . uh . . . brainfart!’.
1996		Eble Sl. and Sociability 51: A brain fart is a ‘temporary loss of memory’ (‘I’m having such a brain fart l can’t think of my own phone number’).
2001		N. Gaiman Amer. Gods (2002) 303: I dunno. I just had a brain-fart or something. It’s gone. Who is it again?
2007		W. Ellis Crooked Little Vein 233: Those episodes of The West Wing where the president has multiple scelerosis brainfarts.