Green’s Dictionary of Slang

a very untidy, messy place, e.g. a child’s bedroom.

1976		(con. 1944) J. Braine Waiting for Sheila (1977) 98: I see your mother’s using the best room again. She’ll make it a proper tip soon.
1989		M. Amis London Fields 336: Dean’s flat was a tip. And so was Dean’s van.
1998		G. Burn Happy Like Murderers 142: It was turning into a filthy tip.
2000		Indep. on Sun. Real Life 20 Feb. 11: I do Pam’s house each Thursday, which is a tip.