Green’s Dictionary of Slang

(US) a braggart, one who ‘swells’ the truth.

1856		Thackeray Character Sketches 77: He has scraped acquaintance with Lord Ballyhooly [...] Sweller Mobscau, and their like (all Captain Rooks in their way).
1901		‘Hugh McHugh’ Down the Line 37: Buck is a sweller. He thinks he strikes twelve on all occasions, but his clock is all to the bad.