in heat

in heat idiom
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]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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