Cestre (1086), from Old English Legacæstir (735) "City of the Legions," from Old English ceaster "Roman town or city," from Latin castrum "fortified place" (see castle (n.)). A post-Roman name; the place was the base of the Second Legion Adiutrix in the 70s C.E. and later the 20th Legion Valeria Victrix, but the town's name in Roman times was Deoua (c. 150 C.E.), from its situation on the River Dee, a Celtic river name meaning "the goddess, the holy one."
variant of suffix -an (q.v.), with connective -i-. From Latin -ianus, in which the -i- originally was from the stem of the word being attached but later came to be felt as connective. In Middle English frequently it was -ien, via French.
<a href="https://www.etymonline.com/word/Cestrian">Etymology of Cestrian by etymonline</a>
Harper, D. (n.d.). Etymology of Cestrian. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/Cestrian