like a bitch in heat. (female behavior) A simile for aggressive sexual behavior or open and assertive sexual desire on the part of a woman, typically used by men about a woman.

The reference is to the willing acceptance of just about any mate by a female dog in estrus. Estrus is the technical term for periodic sexual receptivity found in many mammals, named more colloquially as “heat.” The likening of a woman's behavior to that of a dog's seems calculated to diminish her and, in general, the power of female sexuality and the woman's role in choosing sexual partners. The bitch in heat is not only shamelessly receptive to sex but also implicitly powerless to resist the overtures of even the most undesirable of mates. As Freud notes, “It would be incomprehensible...that man should use the name of his most faithful friend in the animal world—the dog—as a term of abuse if that creature had not incurred his contempt through two characteristics: that it is an animal whose dominant sense is that of smell and one which has no horror of excrement, and that it is not ashamed of its sexual functions.”

In Sexus, the first volume of Henry Miller's memoir, he provides a vivid scene:

It happened so quickly that she didn't have time to rebel, or even to pretend to rebel. In a moment I had her in the tub, stockings and all... I lay back and pulled her on top of me. She was just like a bitch in heat, biting me all over, panting, gasping, wriggling like a worm on a hook.

like a bitch in heat. (motion) To move like a bitch in heat is to move very, very fast.

While this is a much less common use of the phrase, I was able to find several references to illustrate it. In a review of The Blair Witch Project in the online magazine Arrow in the Head, John Fallon he complains about the lack of verisimilitude: “Take the finale for example: being in their shoes I would’ve tossed that camera aside and ran like a bitch in heat!”