The suffix -et may have been added to create a rhyme with muet. The resulting word can be interpreted as an adnominally used past participle form of the verb *surder, 'to deafen', otherwise not attested in Anglo-Norman.
a.
1deafened, unable to hear:
(
c.1275; MS: s.xiii4/4
) Fraunceus ala cum surdet, Nul mot sona cum fust muet
533
surdet (c.1275)