A testicle from a monkey or ape used as a possible means of rejuvenation, either by being grafted on to a man, or by the injection of an extract. Frequently in plural. Also attributive (allusively).
Before the use of monkey testicles was popularized by Serge Voronoff (1866–1951) in books such as Rejuvenation by Grafting (1925) and The Study of Old Age & My Method of Rejuvenation (1926), the technique of organotherapy had been practised in the 19th century using guinea pig and dog testicles.
Early 20th century; earliest use found in The Sheboygan Press. From monkey + gland.