When a lack of sleep/ energy causes a fully functional adult to 'zone-out', become forgetful or impatient, and possibly run in to furniture/ walls.
'Why are you staring at the oven? You've been there for 5 minutes and haven't moved...'
'Sorry, I'm running on retard. Kitty kept me up with her fussiness last night... Didn't sleep a wink.'
Walking out the front door and realizing you forgot your keys, then walking back inside only to forget what you went inside for until you try to lock the front door... AGAIN.
The title given to any person with the uncanny knack for accidentally interrupting folks having sex, generally right before they climax. The type of interruption can range from a phone call to a 'surprise' visit and entails multiple 'incidents' over an extended period of time.
Wife: "OHH F&^K... Just like that, baby!"
Kid K, popping up beside of the bed: "HEY GUYS! What are y'all doing? I wanna play, too!"
Husband to Kid K: "Get out of here! No you can't play with us but if you get out, I'll be out there in just a minute!"
Wife: "Crap... Another orgasm foiled by our little 'coitus interruptus'..."