Libido: 1. Sexual drive. 2. In psychoanalysis, the psychic energy from all instinctive biological drives.
Libido in Latin means "desire, longing, fancy, lust, or rut." Although the adjective libidinous, meaning lustful, has been used in English for 500 or so years, libido only entered the language in 1913, thanks to Sigmund Freud and other psychoanalysts who applied the term to psychic energy or drive, and especially to the sexual instinct.