Sard

While we're at it, the oldest known f*** word in English was <sard>,
which became completely extinct in the 17th c. Its OE ancestor is
attested just once, in Northumbrian, as an imperative (<ne serð>). All
later forms have /d/, and the OE word was likely *seordan, *seard,
*surdon, *sorden (ON serða, sarð, --, sorðenn/stroðenn). It isn't clear
if the Northumbrian example represents an inherited word or a
Scandinavian loan. There are also High German cognates (e.g. OHG sertan*
< *serð-a/i- and serten* (wk.) < *sarð-ija/i:-). Where can it come from?
Any thoughts?

Piotr