"female of a bovine animal," especially the domestic ox, Middle English cu, qu, kowh, from Old English cu "cow," from Proto-Germanic *kwon (source also of Old Frisian ku, Middle Dutch coe, Dutch koe, Old High German kuo, German Kuh, Old Norse kyr, Danish, Swedish ko), earlier *kwom, from PIE root *gwou- "ox, bull, cow."
Applied to the females of various large animals from late 14c. As an insulting or degrading word for a woman, 1690s.
"intimidate, daunt the fear or courage of," c. 1600, probably [OED] from Old Norse kuga "oppress," which is of unknown origin but perhaps has something to do with the Scandinavian forms of cow (n.) on the notion of "easily herded." Related: Cowed; cowing.