her/his face

No idea beyond the obvious. Adequate context, please.

Since this is in the idiom section, do you mean when someone can't remember a person's name and calls them something like "what's her face"? If so, that's what it means.

'What's-his/her-face'! Good idea, Leah.

what's-his-face

Noun. A reference to anyone whose name one has forgotten.

I would add that the "standard,' informal term for someone whose name you can't remember is "what's-his-name." "What's-his-face" strikes me a rather rude or sarcastic variation.

I agree that "what's his/her name" is more standard and sounds more natural to me, but in my experience "face" has become an accepted occasional variant without being definitively rude. It's possibly also a tonal thing.

Than you all. Yes, I meant that.