gwddf.

From Proto-Celtic *wodwos (“spoils”), whence also Old Irish fodb (“spoils”), from the practice of counting captives as "so many necks". *wudwos may be related to Proto-Celtic *wodwo- (“cutting”) (whence Old Irish fodb in the sense of "division"), with semantic shift "cutting" > "weapon" > "weapon-spoils" > "spoils" (though these shifts are not straightforward). If so, then the root ultimately goes back to Proto-Indo-European *wedʰH- (“strike”), whence gwasgu (“to press”), Lithuanian vedegà (“a kind of axe”), Sanskrit वध् (vadh, “to strike, slay”), Tocharian A wac (“struggle”). The variant gwddwg is cognate with Old Breton guodoc (Middle Breton gouzouc, Breton gouzoug, goûg), but it is difficult to explain the alternation between -wg, -w, and -f. The oldest form of the word cited in Geiriadur Prifysgol Cymru is the Middle Welsh plural form gedueu (13th century; = modern gyddfau), while the soft mutation forms wðwc, wddf, and wddw are all attested in the 14th century, indicating that the alternation is very old.