Green’s Dictionary of Slang

(S.Afr.) involved in a relationship or infatuation.

1958		D.A. Stewart in Petersburg Eng. Medium School Mag. 72: Though now I know the ways of girls, / I have not found my taste. / The one I loved is in my mind / But she, alas, is cased [DSAE].
1963		A.M. Louw 20 Days that Autumn 32: All the girls in my class are cased [...] They say I am a square, that’s why the boys don’t ask me.
1977		C. Hope Ducktails in Gray Theatre Two (1981) 54: How can we get cased? We don’t even know each other.