running on empty - idiom lacking energy or enthusiasm He's been running on empty for months now - a vacation will do him good. The program to build low-cost housing has not attracted much support and is running on empty.

This phrase is often aplied to cars. The gas tank sensor will detect how much fuel is left. When you're driving with the indicator on empty, you say "I'm running on empty". This also applied to people. If you are exhausted and feel like you can't go on much longer, you can sat "I'm running on empty."