Definition
A bill consisting of many riders that attracts many, often unrelated, floor amendments, which may provide special benefits to various groups or interests.
Origins
From Christmas tree + bill.
How People Actually Use It
an initially minor bill in the senate that attracts add-ons and/or changes until it begins to "look like it has something for everyone"
"The proposed legislation was looking more like a Christmas tree bill every day."
Source: Urban Dictionary