on_the_sly
1. [ adverb ] in a furtive manner
Synonyms: furtively
Examples:

"the soldiers were furtively crawling through the night"

Related terms: furtive furtive
Similar spelling:   on_the_spot
  on_the_side
  on_the_QT
  on_the_go
  on_the_job
  on_the_Q.T.
  on_the_hook
  on_the_quiet