To puncture, to brand (Greek, stigma, a puncture). Slaves used to be branded, sometimes for the sake of recognising them, and sometimes by way of punishment. The branding was effected by applying a red-hot iron marked with certain letters to their forehead, and then rubbing some colouring matter into the wound. A slave that had been branded was by the Romans called a stigmalic, and the brand was called the stigma.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894