Definition - What does Tanky mean?
If a wine is described as "tanky," this means that it has picked up certain elements of its aging vat, especially those aged in cement or older, neutral wooden vats. It is not a positive adjective and it signifies the wine has a flaw. Most will note that the wine is "dull" and without much aroma, flavor and/or structure.
WineFrog explains Tanky
It is beneficial to age most wines in barrels, stainless steel tanks or barrels and even in cement vats. This can be beneficial for the wines to pick up more character via microoxygenation, resting on their lees, etc. It can add more depth to the nuances of the wine from its perfume to its texture and much more.
However, if a wine is aged too long in their selected aging vats, especially without the proper maintenance, like racking and stirring, a wine can lose character. Over-aging wine can give it a "tanky" character, meaning it has lost any positive characteristics. This is not sometime which can be reversed.