Pahash was a Klingon alias used by Miles O'Brien on a mission to expose a Founder at the highest levels of the Klingon Empire.
"Pahash" was a member of the House of Konjah. When the Changeling Martok recognized his face, but couldn't place it, "Pahash" claimed to have fought at the Battle of Mempa. This placated "Martok" temporarily, after which "Pahash" requested to share a drink with him. (DS9: "Apocalypse Rising")
According to the Star Trek: Deep Space Nine Companion (p. 364), Colm Meaney was so uncomfortable in the make-up and complained so much that Michael Dorn asked the producers never to put Meaney in makeup again.