Today it means: A phenomenon in animal species whereby males (and to a lesser extent females) exhibit renewed sexual interest if introduced to new receptive sexual partners, even after cessation of sex with prior but still available sexual partners. Full definition →
First used in a 1955 publication by behavioral endocrinologist Frank A. Beach, to whom it was suggested by a student, in reference to a joke about US president Calvin Coolidge and sexual promiscuity.
Etymology from Wiktionary, CC BY-SA 4.0.