From the former rite of confirmation (as found in the 1885 Pontifical):
"Next [the bishop] lightly slaps him on the cheek, saying:
Peace be with you."
(note: the word translated as 'slap' could also be translated as 'beat', 'cut', or 'slay'. And who knows how they interpreted that in the craziness that was the pre-Trent world?)
As an aside, since the tradition shows that slapping can be a legitimate sign of peace...