So what does it truly mean, and where does it come from?
(Of course, there will be an instructional illustration.)
1. A low neckline on a woman's garment, especially a dress.
2. A dress with a low neckline in front.
3. The area of a woman's body that is exposed when a garment with a low neckline is worn.
[French, from décolleté, having a low neckline; see décolleté.]
So there you have it, definitively. Now for the illustration.
I knew there was a reason I liked this word so much. Thanks to Michelle Keegan for
the visual demonstration of its meaning.