Green’s Dictionary of Slang

(Aus.) an unsophisticated man, usu. identifiable due to the high waist of his trousers.

2001

Sydney Morning Herald (Aus.) 6 Jan. n.p.: So here’s a tentative guide to Sydney teenspeak: [...] Harry High Pants (an uncool boy or man who is usually identified because he wears his trousers or jeans above the hips).