slip in

verb3

1. •
   To include (e.g. a certain word or phrase) into a sentence discreetly.
2. •
   To enter discreetly.

   • The visitors we do not encourage are the many small boys who slip in unobserved and are a danger to themselves and the staff. — 1960
3. •
   To play a subtle pass into someone in a goalscoring position.