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.