fọnfọn noun /fɔ̃̄.fɔ̃̄/ spelling unconfirmed
Definitions
  1. soundly; deeply; sound asleep
    Wọ́n ń sùn fọnfọn. They're fast asleep.
    Mo sùn lọ fọnfọn ní kété tí mo délé. I fell fast asleep as soon as I got home.
Source: Wiktionary, where this word is spelled “fọnfọn”. Reference: en-fọnfọn-yo-noun-72tmg543