brain fart

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A lapse in concentration, as when one loses one's train of thought.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic, informal A lapse in the thought process; an inability to think or remember something clearly.
• noun idiomatic, informal Something ill-considered and said or done impulsively.