Green’s Dictionary of Slang

(US) excellent! wonderful!

1933		R. Chandler ‘Blackmailers Don’t Shoot’ in Red Wind (1946) 105: It’s nice of you to be sorry about Landrey. Swell.
1949		F. Brown Dead Ringer 67: ‘Swell,’ I said, ‘Thanks a lot.’.
1953		J. Thompson Savage Night (1991) 71: ‘How far you want to ride?’ ‘As far as I can,’ I said. ‘Swell.’.
1965		(con. 1930s) R. Wright Lawd Today 117: ‘It’s on me!’ ‘Swell!’.