Old English heahfore, West Saxon; Northumbrian hehfaro, heffera (plural), of unknown origin, not found outside English. The first element seems to be heah "high," common in Old English compounds with a sense of "great in size." The second element may be related to Old English fearr "bull," or to Old English faran "to go" (giving the whole a sense of "high-stepper"); but there are serious sense difficulties with both conjectures. Liberman offers this alternative:
In modern use, "a female that has not yet calved," as opposed to a cow, which has calved, and a calf, which is an animal of either sex not more than a year old. As derisive slang for "a woman, girl" it dates from 1835.