behind someone's back
Out of one's presence or without someone's knowledge, as in Joan has a nasty way of maligning her friends behind their backs. Sir Thomas Malory used this metaphoric term in Le Morte d'Arthur (c. 1470): “To say of me wrong or shame behind my back.” [Early 1300s]
Words nearby behind someone's back
behest,
behind,
behind bars,
behind closed doors,
behind in,
behind someone's back,
behind the eight ball,
behind the scenes,
behind the times,
behind time,
behind-the-scenes