If I remember well, "flandrien" started out as a mildly derogatory term for East and West Flanders seasonal workers in northern France and Wallonia, then became a honourary term for Flemish track and road cyclists.
Wikipedia calls it a belgicism - an example in Belgian French is the word "dringuelle" (drinkgeld in Flemish, tip in English, pourboire in French) or in Flemish the word kado (cadeau in French, gift in English).
Today a Flandrien denotes "a cyclist with an undiluted attacking instinct. A cyclist who rides through all conditions, never hangs his head (and preferably also wins the race)." Some of the overtones of regional superiority are not great, but okay.
I hope our Flemish readers will forgive our boldness in using one of their very own terms to name one of our products. Well, at least it is a neat and classic
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