Officially it's an enema to flush out the colon but in the slang it means to literally scare someone shitless, which has the same effect as an enema.
Chrissy in the Sopranos got a high colonic when Junior's thugs took him out to the Meadowlands for a mock execution in season 1.
What you tell somebody you hate to order at the bar.
dirtbag: Uhh, what should I get?
you: How about a nice high colonic for you and your friends? Bartender! Three high colonics!