Someone who puts drinks on someone elses tab, unbeknownst to the person responsible for the tab payment.
To Bartos: Verb
To put drinks on other peoples tabs (generally friends) without their knowledge. The act of being cheap and untrustworthy in a bar situation.
Origin: Youngstown, Ohio.
You: $200! I only bought 6 drinks!
Bartender: Your friend that just left was putting the drinks on your tab.
You: He just bartosed the shit out of me...