"young cow that has not had a calf," Middle English heifer, from Old English heahfore (West Saxon); Northumbrian hehfaro, heffera (plural), "heifer," of unknown origin, not found outside English.
The first element seems to be heah "high," which is common in Old English compounds with a sense of "great in size." The second element might be from a fem. form of Old English fearr "bull," from Proto-Germanic *farzi-, from PIE root *pere- (1) "to produce, bring forth." Or it might be related 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:
Old English seems to have had the word *hægfore 'heifer.' The first element (*hæg-) presumably meant 'enclosure' (as do haw and hedge), whereas -fore was a suffix meaning 'dweller, occupant' ....
In modern use, a female that has not yet calved, as opposed to a cow (n.), which has, and a calf (n.1), which is an animal of either sex not more than a year old. As derisive slang for "a woman, girl" it dates from 1835.