Tomber sur un os.

>>> It was at WWI in 1914 the soldier couldn’t get enough food. The most soldier only got the bone. Because it is hardly to eat the bone. The phrase means ‘ someone has a big problem or it is difficult’. Par exemple, ‘C’est trop difficile. C’est tomber sur un os pour moi.’

Tomber = full

sur = on

un os = a bone