When the brain has turned from sharp crystal to something with all the clarity of cotton wool. A time when the mistakes you make really are stupid, instead of the more sophisticated ones you make when you're the ball -- the ones that require concentration. Where making a photocopy involves spilling toner for no reason you can explain afterwards. It's also, though this is not part of the definition, the state where early afternoon seems to last as long as February, at least in my experience. The word is, contrary to all that is good and right in the world, obsolete, except as a character name in George Eliot's Middlemarch:
“People often pretend to tell me things, when they might as well say, 'Mawmsey, you're a fool.'”
Okay, not the best usage example ever, but I am mawmsey myself this morning and a better one is not coming to me. Maybe I'll find one at the bottom of another cup of coffee . . .