on the sly

1. Furtively, secretly, as in She's always eating cookies on the sly. The adjective sly, which means “cunning” or “crafty,” is here used as a noun. [c. 1800]

Related Words

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