Bitcoin Days Destroyed or BDD, is a metric that refers to the time since the last movement of coins.
The metric is derived by determining the number of coins in a transaction multiplied by the number of days since the transaction was included in a block.
In theory this metric could be a mild threat to fungibility. For example, when Bitcoin miners prioritize transactions by their BDD, they treat some coins preferential treatment, therefore those coins are more valuable.
The metric is also widely used as a currency-wide indicator to indicate if long time holders are moving their cons.