(UK) 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 First Lord's daughter proud,
Snubbed earls and viscounts nightly—
She sneered at barts aloud.
And spurned poor Joe Golightly. 1867, W.S. Gilbert, Joe Golightly: