Green’s Dictionary of Slang

fucking adv.

1. [mid-19C+] (also fracking) a general intensifier, totally, completely, e.g. fucking stupid.

2. [1970s+] used, usu. in the negative, to imply definitely (not), certainly (not), e.g. fucking thanks.

In phrases

fucking well (adv.) (also shagging well)

[1910s+] generally used for emphasis, absolutely, very well, very much, extremely.