To modify your street car with go-fast race parts or other setups which hold little to no value to the driver, but is good for posing. This is especially popular in the BMW E30 community. Similar to rice, except trying to look like a track rat instead of a street racer.
btw, it can be used sarcastically by people who are actually upgrading their stuff for good reasons in jest towards the people who do it solely for looks.
Man I just deleted my high beams!!! And took out my passenger seat!