Green’s Dictionary of Slang

soft n.

1. a weakling.

[US]J.C. Neal Charcoal Sketches (1865) 165: ‘Soft’ being the positive announcement of a good easy soul, and ‘saft’ intimating that his disposition takes rank in the superlative degree of mollification.
[US]S. Northup Twelve Years A Slave 234: He must take me for a soft, to think he can come it over me with them kind of yarns.
[UK]‘George Eliot’ Adam Bede (1873) 82: If you’ve got a soft to drive you: he’ll soon turn over into the ditch.
[UK] ‘’Arry at the Play’ in Punch 2 Nov. in P. Marks (2006) 40: I’m aware there is softs as prefers to see Virtue wop Vice.
[UK]W. Pett Ridge Mord Em’ly 2: What d’you take me for, Ginger? [...] A soft?
[US]E.H. Babbitt ‘College Words and Phrases’ in DN II:i 62: soft, n. A silly person.
[US]Monteleone Criminal Sl. (rev. edn).

2. the stomach, i.e. the ‘soft’ part of the body.

[Aus]Bell’s Life in Sydney 26 Feb. 1/4: Jack holed Tom in the soft, and he’ll hole you.

3. (US black) a woman, a girlfriend.

[US]Cab Calloway ‘For the Last Time I Cried Over You’ [lyrics] Oh, man, that soft cut out on me. / What you sayin’? / Yes, she copped a final on me.
[US]Pittsburgh Courier (PA) 10 Feb. 7/1: The males were in [...] skimmers, kilt ties and appendages. The softs were in fine [...] swishes with accessories.

4. see soft money n.

In phrases

come the soft (v.)

(Aus.) to seduce.

[Aus]Bell’s Life in Sydney 17 Oct. 2/2: Ann Handley [...] was accused of coming the soft over an Adonis bearing the classic patronymic of Suang [...] and easing him of 5s.
do the soft (on) (v.)

1. to make love.

[Aus]Bulletin (Sydney) 7 Feb. 7/1: The cock that you see at his feet was a friend / Named Alectryon, a chap ever ready to lend / Assistance in watching whilst Venus and Mars / Were doing the soft until one night, my stars!

2. to flatter.

[UK]J. Manchon Le Slang.