(UK, archaic, slang) A die, especially one that is loaded.
"Oh," said Killigrew, "you had the wit not to give yourselves out for sharpers?"
"Yes, sir; we used common dice; not tatts."
"You'd best keep the tatts till he's drunk," said Killigrew drily, "for I think you'll find my lord has a head on his shoulders when he's sober." 1897, Frances Heath Freshfield, The Wrothams of Wrotham Court, page 73: