Back in the day, a bookie  was short for bookmaker  or someone who determines gambling odds and pays off bets. Over time, the slang term started began describing a shady person who bets on sporting events.

A bookie is someone who facilitates gambling by setting odds, accepting and placing bets, and then paying out the winnings. It's short for the term bookmaker. A bookie was a slang term more often used when online sports betting wasn't readily available.