IPA: /ˈstʌfər/
KK: /ˈstʌfər/
An item that is inserted into something else, often used for advertising or as a small gift, like a toy or candy placed in a stocking during holidays.
She included a small toy as a stuffer in the children's Christmas stockings.
Stuffer is formed from "stuff" (from Old French *estoffer*, meaning to pack or fill) and "-er" (meaning a person who). A stuffer is a person who fills or packs something.
Think of someone who is 'packing' ('stuff') something — that's why a stuffer is a person who fills or packs.