Freebie means free things. Why is there a post-fix -bie? Does it have meaning itself?
The slang word freebie is an example of a rhyming reduplication, which is the term for a word that repeats (possibly with modification) a part of the word stem at another place in the word. English rhyming reduplication often places the reduplicated part at the end of the word. That part does not generally have any real meaning. Other examples of rhyming reduplication include: