As an adjective it describes person/situation presenting strong irony/paradox.
As a 'descriptive phrase' usually appears as Fuck. Unbelievable. (immediately followed by that is) as a *strong* comment on a paradoxical/ironic situation.
Artur, you are fuckunbelievable... You are able to say 'yesterday'
that catalan language was banned until Franco's death and today you
change the discourse for saying that what was "violently" banned.
You're Fuck'unbelievable !!!