lurky vs on the sly

lurky

adj
  • Tending to lurk; given to or indicative of lurking 

on the sly

adv
  • Slyly, in an inconspicuous manner, so as not to be seen; secretly; stealthily. 

How often have the words lurky and on the sly occurred in a corpus of books? (source: Google Ngram Viewer )