hide a dagger in a smile -- with murderous intent behind one's smiles; a murderous heart under a smiling exterior; a smile of treachery; cherish [nourish; warm] a serpent [snake; viper] in one's bosom; conceal a knife in one's smile; conceal a sword in a smile -- injure people under the cover of mildness; cover the dagger with a smile; daggers hidden behind smiles; feline amenities; the knife behind a smile; There is a hidden dagger behind (his) smiling face.; treachery lying underneath a mask of friendliness; trip sb. up while smiling all the time; utter honeyed words but carry a lethal weapon in one's sleeve; Velvet paws hide sharp claws.; When the fox preaches, take care of your geese.