Translates literally as "no problem" but interpreted in English as "relax" or "take it easy" (also written Momentai)
Its a Cantonese phrase made famous by the Bandai show Digimon Tamers, by the Digimon character Terriermon, who it has almost become a trademark phrase.
It's really written Momantai or Moumantai, but stupid monolingual Anglos with no hearing that speak with a potato in their mouth will keep writing "Momentai all over the internet.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"