Where does “fodb” come from?

fodb (Middle Irish) comes from Old Irish odb, from Proto-Celtic odbos, from Proto-Indo-European h₃ésth₁ — bone.

Ancestry of “fodb”, step by step

Step	Language	Word	Meaning
1	Old Irish	odb	knot; lump, swelling, protuberance; difficulty,...
2	Proto-Celtic	odbos	—
3	Proto-Indo-European	h₃ésth₁	bone

fodb — Etymology, Origin & Word History | Etymology Explorer