1
                           general:: 
                            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]
                        
                        
 
                        
                            American Heritage Idioms
                        
                        
                    
                    
                        2
                           general:: 
                            Phrase(s): on the sly 
Fig. secretly and deceptively.  •  She was stealing little bits of money on the sly.  •  Martin was having an affair with the maid on the sly.
                        
                        
 
                        
                            McGrawhill's American Idioms And Phrasal Verbs