Definition - What does Fat mean?
A wine described as "fat" is typically one that is high in alcohol and full-bodied; its aromas, flavors and textures would also be bold and rich. This may describe a big wine, even a young wine, but it may also be an indicator that the wine is out of balance and/or needs some time to age and gain finer subtleties with time.
A fat wine is the opposite of a flat wine.
WineFrog explains Fat
"Fat" is a descriptor used to say that a wine is bold and/or rich. These wines are often higher in Alcohol by Volume (ABV) ratio with very enhanced and magnified aromas, tastes and textures. Depending on the context and style of such a wine, it may be an expected trait or a somewhat undesirable one.
A Port, for instance, is a fat wine. But this is the nature of Port, as these are wines in which additional alcohol is added to make them hot. The grapes used to make this wine are also rich by nature, with a lot of depth and character. The same might be said for Marsala, Madeira and other fortified wines.
Other "fat" wines, while they may be unpalatable for some, might simply be young wines with a lot of structure and character, which can be overwhelming for the consumer. Sometimes these wines just need aging in proper conditions in order for them to become more refined with time.