Green’s Dictionary of Slang

(UK juv.) a euph. substitute for fuck n. in various contexts.

1986		S. King It (1987) 706: I don’t know what the frock Haystack’s up to down there. [Ibid.] 823: They wouldn’t see a frockin thing.
2001		OnLine Dict. of Playground Sl. 🌐 frock! n. Substitute word used in place of ‘fuck ’ as in ‘What the frock was that?’.