grinfucking
grinfucking – the act of being (pretending to be) really nice to someone’s face and then turning around and stabbing them in the back
past tense: “grinfucked”
related: “grinfucker” – one who commits such acts
Laura told me she loved my outfit, then told Mike I dressed like a whore; I can’t believe she grinfucked me like that. What a grinfucker.