Green’s Dictionary of Slang

(UK juv.) a general negative, use varying as to context; also as n.

1987		G.A. Fine With the Boys 167: Ish is the Minnesota equivalent of ick or yeech [...] – mild disgust. [Ibid.] 169: Ish, int. Exclamation of disgust.
2001		OnLine Dict. of Playground Sl. 🌐 tish n. Tish is the slang for cuntish and can be used in a variety of ways. Something or someone can be tished, or can look tish, equally, you can also tish yourself. It’s a great word that is extremely versatile and can be said to cause maximum confusion with out offending anyone.
2003		Eble Campus Sl. Nov. 4: ish – euphemism for shit: I can’t believe that ish.