Definition - What does Dumb mean?
Dumb is a descriptor used to describe a wine which is closed, meaning the wine does not or cannot exhibit any sensory characteristics. This characteristic may be due to the wine needing some extra aging in the bottle in order for the wine to mature and show more characteristics, however, there is no exact evidence of why it occurs.
WineFrog explains Dumb
A dumb wine which does not show signs of expression of fruit, terroir or other characteristics may be fixed with extra bottling or long decanting in a decanter which will give the wine significant exposure to air. The time in which a wine may be dumb is not definitive and may last for many months or many years. In some cases, these wines may never be "fixed", as their integrity of fruit may break down long before other levels of complexity have the time to mature.