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.
on the soft

(Irish) living a life of hedonistic ease.

[UK]letter in Mirror of Life 14 Sept. 14/3: [Irish author] Just a line to lot you know how boxing is kept up in Dublin, and the free way the Americans throw their dollars away when they come here and get them on the soft.