Green’s Dictionary of Slang

soft n.

1. (UK und.) counterfeit notes.

[UK]Pierce Egan’s Life in London 11 Mar. 886/2: [He] had long engaged in the uttering of forged 5l. notes, and of counterfeit sovereigns; and he stated that for the former, which were called ‘softs,’ he gave 35s. each; and for the latter, which were called ‘yellows,’ he gave 7s. each.

2. a weakling, a potential victim (for confidence tricks, prostitutes, etc).

[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.
T.B. Reed Willoughby Captains (1887) 21: ‘He advised me to give it up [...] he made out it wasn’t honourable to use cribs,’ said Telson. ‘Grandmother!’ snarled Parson. ‘Why, Telson, I didn’t think you’d have been such a soft!’.
[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).

3. 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.

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

[US]Cab Calloway ‘For the Last Time I Cried Over You’ 🎵 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.

5. (UK drugs) a narcotic in powder form.

[UK]G. Krauze What They Was 49: I’d recently bought some coke and strated trapping so before leaving [...] I make sure I’ve got the soft cheeksed properly.

6. 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 soft (v.) (also pass the soft, shove the soft)

to pass counterfeit notes.

[US]Nat. Police Gaz. (NY) 16 Oct. 51/3: We left the parties on the day after ‘passing the soft’ [...] the town was in a buzz of wonder at the magnitude, extent and audacity of the fraud.
[UK]Peeping Tom (London) 32 128/2: [G]entlemen engaged in the soft (forged notes), the hard (smashing), ramping, sneaking, doing a panny, making a reader, or picking up a cat and her kittens — the cat being a quart pot and the kittens pints!
[US]Nat. Police Gaz. (NY) 25 Oct. 76/1: [headline] A New Mode of Shoving the Soft.
[UK]Peeping Tom (London) : .
[UK](con. 1840s–50s) H. Mayhew London Labour and London Poor III 386/2: I would cut that game of ‘smatter-hauling’ [...] and do a little soft,’ (pass bad notes).
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.