Know full well Definition 1 of 2Pro Tip 1/3Feigned Ignorance Used when someone pretends not to know something they actually do. You know full well why I was upset, stop pretending. Pro Tip 2/3Emphasizing Certainty Emphasizes strong certainty about something, leaving no doubt. I know full well we made the right choice. Pro Tip 3/3Sarcasm or Reproach Can express sarcasm or reproach when the speaker thinks the listener is pretending ignorance. Oh, you know full well what you did wrong!