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In a state of sexual excitement and able to breed, as of a female animal. Do you hear the stray cat howling and screeching at night? She must be in heat.
in a period of sexual excitement; in estrus. (Estrus is the period of time in which females are most willing to breed. This expression is usually used for animals. It has been used for humans in a joking sense. *Typically: be ~; get [into] ~; go [into] ~.) Our dog is in heat. She goes into heat every year at this time.
In a state of sexual excitement immediately preceding ovulation. For example, Our cat's in heat so we have to keep her inside. This expression applies to most female mammals and indicates the period when the animal is fertile and most receptive to mating. [Mid-1700s]