Abbreviation or diminutive for baronet, later replaced by Bt. It is a post-nominative title; the pre-nominative is Sir (e.g. Sir John Smith Bart.).
"The distinguished author signed his book as "John Anderson Bart" to indicate he held the hereditary rank of baronet."
In plain English: A bart is an old-fashioned term for a person who works behind a bar serving drinks to customers.
"He stopped at the bar to order a beer and have a chat with his friend."
A diminutive of the male given name Bartholomew.
"When we arrived at the BART station, the train was already pulling away from the platform."
Initialism of Bay Area Rapid Transit.