to slip something into something else

Example 1:

Johnny and I went for coffee last night after class. He tried to pay for my drink, but I insisted on treating him. When I got home, I found $10 in my jacket pocket! Johnny had slipped money into my pocket! He paid for the coffee after all!

Example 2:

There was a special news report on TV. The reported was talking about something dangerous that could happen in bars and nightclubs. When you go to a bar or a nightclub and a stranger wants to buy you a drink, be careful! They could slip a drug into your drink without you knowing! You could get sick or worse! The report advised to use your best judgement before accepting drinks from people you don't know.

Meaning:

slip something into something else means to secretly put something into something else. They are usually trying to hide the action in order to surprise someone or to be sneaky.

For example :
He slipped the money into my pocket.
He secretly put the money into my pocket. I didn't know he did it.

This idiom was taken from the LSI textbook "Reading Transitions" which is used at LSI schools to teach Level 4 Reading/Vocabulary. For more information, please visit www.languagesystems.com