One of the justices, i.e. judges, on the *county circuit, hearing cases at local courts. These justices ensured that criminals with local influence and able to intimidate local courts were subject to the king's justice, which was not to be imposed upon. The Latin for justice-in-eyre is justicarius in itinere. -
Cf. Eyre
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.