from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Having a fitted top and a flared bottom: an A-line dress.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Having a silhouette that replicates the letter A by flaring out from the top, like a funnel, narrow and fitted at the neck and shoulders, skimming the hips and widening toward the bottom, with little or no shaping or seaming at the waist.
From garments being shaped like a capital A.(American Heritage® Dictionary of the English Language, Fourth Edition)
From A + line ("figure") (Wiktionary)