Green’s Dictionary of Slang

1. a police van [its mesh-covered sides].

1958		in L.F. Freed Crime in S. Afr. (1963) 127: ‘Hulle het hom gewang met die nylon.’ (He was taken away in a police van).
1960		C. Hooper Brief Authority 209: Because of the appearance of their perspex panels they were soon known as ‘Nylons’; or they were referred to by the city name of ‘kwela-kwela’.
1980		E. Joubert Poppie Nongena 169: They yelled: Nylon, which was what they called the pick-up with the nylon mesh netting.

2. a well-behaved young woman [? her ‘smoothness’].

1986

Sophiatown in M. Orkin At the Junction (1995) 165: I’m a virtuous well-bred girl, I’m a nylon.