Green’s Dictionary of Slang

1. foolishly, stupidly.

1840		T. Haliburton Clockmaker III 58: Talkin’ cute, looks knavish; but talkin’ soft, looks sappy. Nothing will make a feller bark up a wrong tree like that.
1937		J. Curtis You’re in the Racket, Too 37: Don’t talk soft. Love’s a lot of muck.
1950		C. Harris Three-Ha’Pence to the Angel 85: ‘Romantic, perhaps.’ [...] ‘Oh, don’t talk soft.’.
1960		(con. 1920s) D. Holman-Hunt My Grandmothers and I (1987) 151: Don’t talk soft.
1977		A. Bleasdale Who’s Been Sleeping in my Bed 111: Deserve t’be robbed soft, they do.
1999		K. Sampson Powder 122: Don’t talk soft.

2. comfortably.

1909		H. Green Mr. Jackson 202: Talk about fallin’ in soft [...] ain’t it a peach of layout.
1914		T.A. Dorgan Indoor Sports 19 Oct. [synd. cartoon] Well I fell into it pretty soft since I left here.
1931		P. Singer ‘The Electric Warden’ Prison Stories Mar. 🌐 ‘I’m in soft,’ thought Tully. ‘From Balboa Park to a swell job as a warden’s butler.’.

3. (W.I.) absolutely overcome, e.g. have somebody soft.

1946

‘Uncle Newton’ Ups and Downs of Newsy Wapps Bk 2 30: We thought we ‘had her soft’.