Green’s Dictionary of Slang

slippery n.

1. soap.

1839		H. Brandon Dict. of the Flash or Cant Lang. 168: The slippery and the spunks are fenced by leary-coves all day.
1852		Kendal Mercury 17 Apr. 6/1: A precious christian stopping his tatar-trap (mouth) vith slippery (soap), and rolling himself about the pave like a prad (horse) in the bits.
1857		‘Ducange Anglicus’ Vulgar Tongue [as cit. 1839].
1859		Matsell Vocabulum.
1882		Sydney Sl. Dict. (2 edn) 8: Slippery - Soap.

2. the vagina.

2013

N. Williams Unfaithfully Yours 230: It is not just a matter of fishing out the old penis and shoving it up the old slippery.

SE in slang uses

In phrases

do a slippery (v.)

to absent oneself, usu. for dubious purposes.

1975

‘P.B. Yuill’ Hazell and the Three-card Trick (1977) 168: Then I gits the idea of a bent passport under another name — so’s I can do a slippery for a whole week somewhere they’ll niver be.