Definition of brain fart :.
1. (n.) Malfunction of the brain on a given thought process, usually occurring during an important situation.
Notes: Also: cranial flatulation, brain cloud, brain cramp
2. (pron.) When one conveniently forgets something just at the right moment when he or she is asked an important question. In the act of trying not to look stupid as one is they cover it up by indicating that they had a brain fart. Unlike a normal fart, these leave no revolting odor or any clue to their presence.
Example: Bob: "What day of the week is it today, Fred?" Fred: "It's chili day in the cafeteria!" Bob: "No!... I asked what day of the week!" Fred: "Oh, brain fart... It's Friday."