A more thorough explanation:

Definition: Hat money is a small gratuity that is traditionally paid to the master of a ship (and sometimes the crew) for taking care of the cargo. It is also known as pocket money or primage.

For example, if a shipper wants to ensure that their cargo is well taken care of during the voyage, they may offer hat money to the ship's master as an incentive. This payment is separate from the freight charges and is considered the master's right.

The term "primage" is often used interchangeably with hat money and refers to a small compensation or duty paid to the master for their care and trouble as to the goods.

It is important to note that hat money and primage are not covered by the policy of insurance and the master has a lien on the cargo for these charges as well as for the freight.