Why does the professor blush when Elizabeth ask him the difference between decolletage and decollete in The lu?

I looked it up in google translate and it said Decolletage means cutting and decollete is the neckline! THat makes no sense. Why are these people so snobbish! So I just guessed that it was wrong and was wondering if you all knew, the people that speak French better than me or the people who has read it, If you need help, it's on page 35. Merci beaucoup mes amis!