In betting slang, a beard is a stand-in who places wagers for somebody else. The real bettor stays out of sight, much as a false beard hides a face.
People use beards for several reasons. A sharp bettor may want to stop the bookmaker from moving the line or restricting their account. Someone who has been barred may try to keep betting through another person. A public figure, or someone close to a sporting event, may want to stay anonymous, and some bettors use beards to get around betting limits.
Betting through a beard is risky and, depending on the jurisdiction, can be illegal — especially when it hides insider involvement or breaks a ban. For sportsbook and casino operators, beard activity is a recognised fraud and integrity risk, which is why KYC, linked-account detection and monitoring of unusual stakes are standard parts of account management.