in secret

1. Unknown to others, privately. For example, They met in secret, or, as Shakespeare put it in Love's Labour's Lost (5:2): “One word in secret.” [Second half of 1400s]

Related Words

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