Green’s Dictionary of Slang

used as a euph. for fuck n. in various contexts.

2001		Eble Campus Sl. Fall 12: WHAT THE FREAK – exclamation.
2001		OnLine Dict. of Playground Sl. 🌐 freak! n. Substitute word used in place of ‘fuck’ as in ‘What the freak was that?’.
2005		Mad mag. Dec. 37: What the freak is that about, Doc?
2016		A. Wheatle Crongton Knights 7: Their stupid pact not to tell me what the freak was going on.