To catch a Tartar; to attack one of superior strength or abilities. This saying originated from a story of an Irish-soldier in the Imperial service, who, in a battle against the Turks, called out to his comrade that he had caught a Tartar. ‘Bring him along then,’ said he. ‘He won’t come,’ answered Paddy. ‘Then come along yourself,’ replied his comrade. ‘Arrah,’ cried he, ‘but he won’t let me.’—A Tartar is also an adept at any feat, or game: he is quite a Tartar at cricket, or billiards.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Tart * TatNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes