"Le boudin" colloquially meant the gear (rolled up in a blanket) that used to be carried atop the backpacks of Legionnaires.
[1] Another hypothesis suggests that because the Legion accepted no Frenchmen (hence the adjective in its name), a Frenchman wishing to join could do so only by pretending to be a (French-speaking) foreigner, a Belgian.
Since a person wishing to remain anonymous and lie about his identity often turned out to be a criminal wanting to evade the law and a prison sentence, and criminals rarely make the best soldiers, the "Belgians" ended up with a bad reputation.
[citation needed] "Le Boudin" is sung while standing to attention or marching by all ranks of the Foreign Legion.
Consequently, the Legion contingent at the Bastille Day military parade march brings up the rear.