Green’s Dictionary of Slang

slip-slop n.

[SE slip-slop, any form of sloppy mixture, whether food or drink]

1. kissing [echoic of the exchange of spittle].

[UK]N. Ward London Spy XVI 333: Such a wonderful Mess of Slip-Slop lick’d up between Brother Bat and Sister Bet.

2. in pl., as a drink.

(a) a non-alcoholic drink taken for medicinal purposes.

[UK]W. Cowper Letters and Prose (1981) II 441: The remedy is certainly a most detestable affair, but when taken early in the morning, without slip-slops, is attended with less labour.
[UK]Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Slipslops. Tea, Water Gruel, or any Innocent Beverage taken Medicinally.
[UK]Grose Classical Dict. of the Vulgar Tongue (3rd edn) n.p.: Slipslops. Tea, water-gruel, or any innocent beverage taken medicinally.
[UK]Lex. Balatronicum [as cit. 1796].

(b) any form of soft drink, esp. tea; also attrib.

G.A. Stevens Court of Alexander I:i: Shall he, who conquers Nations, / Fribblish submit to such slip-slop Potations? / In spite of Water-bibbers, you and I know, / We must be true to Wine.
[UK]Harris’s List of Covent-Garden Ladies 102: Women [...] generally poison themselves by drams, or throw themselves into consumptions with slip-slops.
see sense 1.
[UK]W. Combe Doctor Syntax, Wife (1868) 245/2: At length the coffee was announced [...] ‘And since the meagre slip-slop’s made, I think the call should be obeyed.’.

(c) (also slippy sloppy) any drink.

[UK]Mr Mathews’ Comic Annual 8: You may give me a little of the slippy sloppy.

3. (S.Afr., also slop) a thong sandal [the noise it makes as one walks].

[SA] informant in DSAE (1996).
[UK]J. Hobbs Thoughts in a Makeshift Mortuary 224: Does he wear slip-slops?
[SA]CyberBraai Lex. at www.matriots.com [Internet] SLIP SLOPS: Getting back to the beach for a moment, these are the rubber thongs or sandals you wear, keeping them on your feet with a thin strap which causes blisters between your big toe and its partner.
[SA] ‘SA English’ on Joburg.org.za [Internet] Slip slops, slops – Sandals or rubber thongs worn to the beach. These usually have a thin strap between the big toe and the toe next to it. Not to be worn with socks!
P. van Maasdyk Never Go Trekking 33: Flea had seen too many sherpas trudging along happily with only a pair of slip-slops between their feet and the harsh surface of the earth.