Green’s Dictionary of Slang

freak n.2

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

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