Tatar - A ferocious or violent person: a tartar.

n.

1. also Tar·tar (tär'tər) A member of a group of Turkic peoples primarily inhabiting Tatarstan in west-central Russia and parts of Siberia and Central Asia.
2. Tartar Any of the Turkic languages of the Tatars.
3. Variant of Tartar (sense 1).
4. tatar A ferocious or violent person; a tartar.

[Persian Tātār, of Turkic origin.]

This is the first we're seeing of this definition #4.