A person engaged in covert advertising for a cryptocurrency. The shill attempts to spread buzz by personally endorsing the product in public forums with the pretence of sincerity, when in fact he is being paid for his services.
When a coin is promising to solve world hunger, it’s being shilled. In other financial markets shilling is illegal due to the possibility of fraud, however it is a grey area as the shill’s activities must be proven to have placed uninformed parties at the risk of loss.
If the shill’s activities only serve to generate buzz they may be considered legal. In the world of cryptocurrencies however, there are no such laws currently.