Put-Pocket

Someone who secretly places an item into someone else's pocket. This is the reverse of a pick-pocket and may be done to anonymously give a friend money or a small gift when they are feeling down. In addition to the pockets of clothes that you are wearing, a put-pocket may also target unattended coats, jackets, and purses at parties or other social gatherings.

I was feeling down until I discovered that a put-pocket had slipped a $20 into my pocket.