Going overboard means to do too much, go over the top in excess.
Example of use: Matt went overboard when he spent so much money on my birthday present.
Interesting fact about Going Overboard
Not surprisingly, the origin of the idiom “go overboard” is nautical. When one falls overboard from a boat into the water, it’s quite a problem; in the same way, going out of bounds in other areas of life is problematic. Interestingly, the “board” in overboard is actually a term for the physical railings found on a boat; if you went overboard, you went over the railings and into the water.
