From Wikipedia - on the origins of the word "Carnival" -

Etymology - The origin of the name "Carnival" is disputed, between those that have argue a link with the Italian word "carne" (meat), and those that argue a link with the word "carrus" (car). The link with carne would suggest an origin within Christianity, while the link with carro with earlier religions.

From carne levare - Those that argue for the origin from "carne", point to variants in Italian dialects that would suggest that the name comes from the Italian carne levare or similar, meaning "to remove meat", since meat is prohibited during Lent.

From carne vale - Folk etymologies exist which state that the word comes from the Late Latin expression carne vale, which means "farewell to meat", signifying that those were the last days when one could eat meat before the fasting of Lent. The word carne may also be translated as flesh, so suggesting carne vale as "a farewell to the flesh", a phrase actually embraced by certain Carnival celebrants who encourage letting go of your former (or everyday) self and embracing the carefree nature of the festival. However, explanations proceeding from carne vale seem to be folk etymologies and are not supported by philological evidence.

From carrus navalis - Other scholars argue for the origin from the Roman name for the festival of the Navigium Isidis (ship of Isis), where the image of Isis was carried to the sea-shore to bless the start of the sailing season. The festival consisted of a parade of masks following an adorned wooden boat, that would reflect the floats of modern Carnivals. Modern Carnival shares resemblances with the Navigium Isidis.

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