flatter: [13] Etymologically, flatter means ‘smooth down or caress with the flat of the hand’. It comes from Old French flatter, in which the original literal notion of ‘caressing’ had already passed into the figurative ‘buttering up’. The Old French verb in turn was based on Frankish *flat, the ‘flat or palm of someone’s hand’, a word which shared a common source with English flat. => flat
