Green’s Dictionary of Slang

(orig. US campus) a general excl. implying that the speaker has lost the thread, forgotten what they were talking about, made a major mental error and all in all lost the power of rational speech and thought.

1989

P. Munro Sl. U. 44: In the middle of the sentence he blanked out — ‘. . . which leads directly to my . . . uh . . . brainfart!’.