The term "fill er up" is typically used to ask someone to fill a container or a vehicle with fuel. However, when we talk about the antonyms of "fill er up", we refer to the act of emptying or making the container empty. Some words that can be considered as antonyms of "fill er up" include drain, empty, deplete, exhaust, evacuate, and void. All these words indicate the removal or depletion of the contents from a container, resulting in its emptiness. So, when we use "fill er up" to ask for fuel, the antonyms of this phrase describe the opposite action, i.e., the removal of the fuel from the container.