cat in heat; cat in season
Refers to a female cat in estrus, often characterized by loud calling and restlessness.
In spring, the neighborhood cats in heat cry loudly.
Compound of 恋 (こい, 'love') and 猫 (ねこ, 'cat'), literally 'love cat'. The exact origin is uncertain, but it likely arose from the association of a cat's mating behavior with the concept of love.