“Fernet” is a generic term for a type of bitter produced by several manufacturers in Lombardy, Italy.
It will be a herb-tasting liqueur meant to be used as a digestif.
“Fernet” generally indicates that the liquor was distilled from beet molasses. Note that this is not the case, however, with Fernet Branca, which is made from a base of alcohol distilled from grapes.
Fernet Branca is the best-known brand. The Trappist Monks of Trefontane also make a Fernet.
Gaspare Campari made a Fernet in the 1860s; he claimed it was made “preparato con la formula dello svedese Fernet” (prepared with the formula belonging to the Swede named “Fernet”.)
Some say that Fernet was first made by a family named Fernet. Others say that it was being made in the 1400s by the doctor of Henry II of France, possibly named “Fernetius.”
Others say that “Fernet” means “cleaned by iron”, in that while being made (they say) a red-hot iron poker was put into it to “purify it”. In Italian and French, “ferro” means “iron”, in French “nettoyer” means “to clean”, and there you have “fer – net”. But this is so obscure as to be easily dismissed.