borrowed from Latin cervīnus, from cervus "stag, deer" + -īnus -ine entry 1; cervus, going back to dialectal Indo-European *ḱer-u̯-os, from a base *ḱer-u̯- "having horns," derivative of *ḱer- "bony material constituting the skull or horns," whence also, from *ḱr̥-u̯-os, Welsh carw "deer, stag," Cornish carow, Breton karo — more at horn
Note: Presumably from the same base is the Balto-Slavic word represented by Lithuanian kárvė "cow," Croatian & Serbian krȁva, Russian koróva, assuming that *ḱ is represented by a centum outcome in this word. Alan Nussbaum (Head and Horn in Indo-European, Berlin, 1986, p. 8) reconstructs a lengthened grade form *kōr-u̯-ā, apparently to avoid positing a laryngeal in order to produce the acute intonation. Old Prussian curwis "ox" would represent the same ablaut *ḱr̥-u̯- as the Celtic word; on the other hand, Old Prussian sirwis "roebuck" appears to have the regular sibilant outcome of *ḱ in Balto-Slavic.