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To understand why, it is necessary to explain where the expressions come from. The origin of thanks is obscure, but there was a time when, when someone received a favor or a gift, it was obligatory to reciprocate. Thus came the “thank you” – and, later, its feminine variation. It comes from the Latin verb obligare, which means «to make responsible», «duty». Therefore, when someone thanks us, it is as if they were saying “I owe you an obligation”. By politeness, those who do a favor respond “de nada”, which means “by no means” and comes from the Latin rem natam. Here in Brazil, regionalism ended up giving rise to new ways of saying the same thing, such as “imagine”, “you’re welcome” and other more informal and recent ones, such as “falou” and “thanks”. Regardless of the version chosen, when we respond to “thank you” with expressions of the kind, we are saying that “by no means are you my debtor”, “you are not obliged to anything”.
CONSULTANCY: Márcio Cotrim, author of the column “O cradle of the word”, in the newspaper Correio Braziliense, and José Augusto Carvalho, doctor in linguistics from Ufes and author of Gramática Superior da Língua Portuguesa
SOURCE: New Frontier Etymological Dictionary, by AG Cunha