Green’s Dictionary of Slang

slip-slop n.

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

1. [early 18C] kissing [echoic of the exchange of spittle].

2. in pl., as a drink.

(a) [early 18C] a non-alcoholic drink taken for medicinal purposes.

(b) [late 18C–early 19C] any form of soft drink, esp. tea; also attrib.

(c) [mid-19C] (also slippy sloppy) any drink.

3. [1960s+] (S.Afr., also slop) a thong sandal [the noise it makes as one walks].