"Bra" is short for "Brassiere," a French word that originally meant "a child's jacket with sleeves." It derives from an Old French word for "arms." Eventually, the word came to refer to the modern undergarment.
Thus, by replacing one word with the other, we go from "a brassiere" (singular) to "a bra" (also singular).
"anties" derives from the word "pants," which was shortened from the original word "pantaloons," another French word. "antaloons" originally referred to a general *style* or *class* of garments either worn in Venice or assumed to have originated there. (That word was further derived from Saint Pantaleon, the patron saint of Venice.)
Again, by replacement, "he's wearning pantaloons" (plural) became "he's wearing pants" (also plural).
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