Welsh slang. The phrase originally comes from the action of having tapped a barrel of beer, draining its contents. The comparison made, then, is between a half-empty barrel of beer and a half-empty brain.
Rory - “Oi nathan, you heard of that tapped cunt called chase”
Nathan - “Yeah mate heard he bin dives outside of matalan for his clobber”
Rory - “Tosser”
Irish slang meaning to be completely broke, out of cash, or having zero money to your name.
While the word is also used in Ireland and elsewhere to mean "crazy," "weird," or out of your mind (as in "tapped in the head"), when used financially it is the ultimate declaration of ruin, painting a picture of a wallet that has been tapped dry.
This directly ties into the related Dublin slang term "tapping," which means actively asking people for money or begging because you are absolutely broke.
"Do you want to head into town for a few pints tonight?"
"No sorry, I can't. I'm absolutely tapped until payday."
"Did you see yer man jumping into the freezing canal?"
"Stay away from him, he's completely tapped in the head!"
in the know...to have the inside scoop...on the cutting edge of knowledge...to know the score...to be on the beam...early adopter knowledge...having a knowledge base that is yet unknown to the masses.
Minorities are tapped in to dangerous synthetic food additives in processed food.