behind someone's back

1. 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]

Related Words

• clandestinely www.thesaurus.com
• covertly 
• furtively 
• personally 
• privately www.thesaurus.com
• quietly 
• stealthily 
• surreptitiously