Many definitions of the term "slutface" mistakenly use slutface synonymously with other terms commonly ascribed for a, what can be generally considered, "loose" woman. But slutface is a title earned for the dame who does more than frequently spread her legs wide open. "Slutface" is certainly used as a worse insult in comparison to the usual "slut."
The phrase that captures the essence of slutface best is
"she takes it in the face."
For those who have differentiated between the common slut and this more particular "slutface" --by recognizing that the "face" portion of the term does, indeed, hold special meaning setting it apart from the normal heap of slut terms--good job.
But one must be careful in the execution and interpretation of this term that she understands the severity of the action, which we can safely describe in verb form as "slutfacing." One can draw from the snideness in which the insult is delivered that slutfacing is not merely giving head.
To be frank, a slutface, who likes slutfacing, is a dame who likes to get face fucked. And this is highly different intensity and psychological state of mind than the typical head giver.
This term should not be used lightly, with the one exception as a term of endearment between the closest of close friends.
"oh, we all know she takes it in the face. slutface."