Crypto slang for a large quantity of a specific cryptocurrency. Alternatively (but less frequently) used to refer to the contents of an individual's crypto portfolio.
In the first definition, there is no generally agreed-upon threshold at which someone's crypto holding becomes a bag, but it is generally considered to refer to a higher-than-average quantity of that coin. Extremely large holdings of a specific cryptocurrency are sometimes referred to as “heavy bags.”
The term is useful in distinguishing between different coins or tokens within an individual's overall portfolio. An investor might have well-performing and poorly performing bags — depending on the market conditions for each asset in question. In these cases, 'bag' is closely related to the term 'bagholder', referring to an investor who does not sell their holding of a specific coin or token, even if it experiences dramatic drops in value.
See also: Bagholder.