Past tense of grub; dug out from the ground, searched for intensely, or worked hard at something difficult.
From Old English 'grobian,' possibly related to 'grub' (a larva or to dig). The past tense adds '-ed' to form the simple past.
The word 'grub' means both a beetle larva AND to dig around in dirt, which sounds gross but comes from the visible larva that appears when you dig—and medieval farmers definitely did a lot of grubbing.
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