ABH AMH were the menacing letters on the pirate flag of the infamous captain Bartholomew Roberts, or Black Bart.
As Roberts bore especial ill will to the small Caribbean colonies of Barbados and Martinique, Roberts flew from his flagship, the Royal Fortune, a black flag adorned with the figure of a pirate, himself, by all acounts, toting a flaming sword in one hand and a dagger in the other, standing on two skulls. Underneath the skulls the letters ABH and AMH were emblazoned:
A Barbadian's Head and A Martinician's Head!
This personal feud against the two colonies culminated in 1720, as Roberts captured the Martinician governor and hung him from the mast. As Bartholomew's infamy and power grew the merchant companies offered huge rewards for his capture or death and ships from Barbados and Martinique proved a particular nuisance to Black Bart, prompting the creation of the flag.
The flag of Bartholomew Roberts is just one of the many variations of the pirate flag, the Jolly Roger.