What is Fmil?
1.
Fucking Mother-In-Law.
For when your vitrolic loathing needs to spill forth, but in a tastefully discreet acronym.
"Dude, she's such a bitch, I think she's ready to become the FMIL."
See
2.
Future Mother-In-Law.
The non-dirty (and probably more oft-intended) version of FMIL.
"I hate getting emails from my FMIL."
See