Green’s Dictionary of Slang

1. (also longers, longs) long trousers.

1952		(con. 1910s) ‘Harry Grey’ Hoods (1953) 40: Dominick [...] had his brand-new church-going longies on.
1965		L. Dunne Goodbye to the Hill (1966) 38: I felt all grown up in my first pair of longers.
1981		(con. c.1920) P. Crosbie Your Dinner’s Poured Out! 97: The short trousers of the Twenties were worn by all boys and getting into ‘longers’ signified that a lad had reached the first step to adulthood.
1993		R. Doyle Paddy Clarke Ha Ha Ha 115: Martin wore longers in the summer.
1995		A. Higgins Donkey’s Years 97: Something light as a feather tickled my bare hairless leg in my first pair of longers.
1999		K. Sampson Powder 395: His shorts were neverending. They were longsjoke? not actually trousers.
2001		G. Coughlan Everyday Eng. and Sl. 🌐 Longers (n): long trousers.

2. (orig. US) long woollen winter underwear, combinations.

1957		M. Shulman Rally Round the Flag, Boys! (1959) 193: ‘Where are you going?’ ‘Home [...] to pack my longies.’.
2001		S. King Dreamcatcher 75: He’s got a nice heavy coat and he’s wearin longies.