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.
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.