This phrase can mean a lot, perhaps to much. In some circumstances it can be the same as 'have it' but of course
'having it' is somewhat in the present tense.
So perhaps think of 'having it' as a outburst
response to having extreme fun, mostly summoned for circumstances of a sexual, drug consuming, party time
Fucking sweet to the eye balls buddy, I'm having a great time in fact you could say I'm fucking 'Having It' !
I guess I should get my ass in gear, strike up
and start fucking 'having it'
! I heard you had a proper session last night with the local hooksta!
lol, yeah she was fucking having it.