Shilling in the crypto sense is covered in full under shill or shilling, which explains the tactics, examples and legal issues involved. This page explains the word on its own, which is often used as a verb ("stop shilling your bag").
To shill a coin is to talk it up loudly and repeatedly, usually on social media, forums and messaging groups, in order to push up demand. A shill may be paid by the project, hold the token and hope to sell to new buyers, or simply be an enthusiastic fan. The word comes from older slang for a person who poses as an ordinary customer to lure others, such as a fake bidder at an auction. In crypto it is used both as a serious accusation and as casual banter about anyone promoting their favourite token.
Shilling becomes a legal problem when promoters hide that they were paid or that they plan to sell. In the US, the SEC has fined celebrities for promoting tokens without disclosing payment, and paid promotion is a common feature of pump and dump schemes. Coordinated shilling by paid influencers and key opinion leaders can amount to market manipulation.
The word is unrelated to the shilling, a currency unit used historically in Britain and today in Kenya, Uganda, Tanzania and Somalia.