Appearance and Character
Shitmouth has grey hair and is grizzled, with a thick beard.
A Clash of Kings
Shitmouth is one of Ser Gregor Clegane's men-at-arms and is part of the group that captures Arya Stark, Gendry, and Hot Pie in the riverlands. Arya does not add him to her prayer kill list as he displays a lack of cruelty in comparison to his comrades.
A Feast for Crows
Rafford informs Jaime that some time ago Sandor Clegane, accompanied by a boy, killed Polliver, the Tickler, and a pimply squire at the crossroads inn. When Jaime asks if they sent men went after Sandor. Shitmouth replies he never liked Polliver and that Sandor is the brother of their commander, Gregor.
Shitmouth declines Ser Bonifer Hasty's offer of land in return for service, as Gregor had promised rich rewards to his followers. According to a semi-canon source, Shitmouth joins Jaime's retinue as it heads for the siege of Riverrun.
Quotes by Shitmouth
Jaime: Who might you be?
Shitmouth: Ser used to call me Shitmouth, if it please m'lord.Jaime: Charming.
—Jaime Lannister and Shitmouth
Bonifer: Any man who remains with me shall have a hide of land to work, a second hide when he takes a wife, a third at the birth of his first child.
Shitmouth: Land, ser? Piss on that. If we wanted to grub in the bloody dirt, we could have bloody well stayed home, begging your pardon, ser. Rich rewards, Ser said. Meaning gold.—Bonifer Hasty and Shitmouth
Quotes about Shitmouth
Though the one they called Shitmouth had the foulest tongue she had ever heard, he'd give you an extra piece of bread if you asked, while jolly old Chiswyck and soft spoken Raff would just give you the back of their hand. 
You heard the man. Find a nice long spear, and shove it up his arse.