Décolletage is a fashion word we kept from French. It is the low opening at the top of a dress or top — the neckline that shows the neck, shoulders, and upper chest — and it can also mean that area of skin when a neckline shows it. Magazines and designers use it; most people just say “a low neckline” or “a low-cut dress.” It is not a rude word in fashion writing, though it talks about a part of the body some students feel shy naming. It belongs mainly with women’s evening or formal clothes, not a sports vest or a man’s open shirt. Though actually, English speakers use it two ways at once: for the cut of the clothes, and for the bit of the body the cut shows. Both are normal in fashion English. Careful English keeps the French mark on the e; everyday English sometimes drops it. You will hear a French-ish ending, a bit like “tahzh.”