Grub is a slang term for food. Australians use it a lot. Example: Come and get your grub.
According to the net, the source of 'grub' is the old Germanic word meaning to dig (which is also the source of grave). The verb 'to grub' came first in English, around 1300, and meant just what it still does: to break up the surface of the ground or to clear the ground of roots and stumps. Derived from it, is the adjective 'grubby' for somebody or something that is dirty and the Australian slang term 'grub' is also for a person who is unclean or who has messy habits. Example: Don't be such a grub. Wash your hands before you eat.