The story
Maglificio Calzificio Torinese (MCT) is a sock and underwear company from Turin established in 1916. It had a production problem with its Aquila brand in 1956. When the problem was fixed, all the new products were stamped with a K, standing for German Kontrolle. Sales surged and in 1967 the K line was formally called Kappa, the Greek letter for K.
The brand has been the shirt sponsor of many football teams in its history, including Juventus, AEK Athens FC, AC Milan, and Barcelona FC, and currently sponsors teams such as OGC Nice, Genoa FC, and Kaizer Chiefs F.C.
From Wikipedia’s article on Kappa (brand), shared under CC BY-SA 4.0.4