ON THE SLY

sly  adj having or showing a cunning and deceitful nature; clever at tricking people or at secretly doing unfair or dishonest things

   A sly, manipulative person

on the sly    done secretly, especially because you know you should not be doing it

   I’m supposed to be on a diet, but I still have the odd bar of chocolate on the sly.

♫  BBC News bulletin 21st January ♫