errant

Straying from the proper course or standard, or outside established limits.

In that there are just seven Planets or errant Stars in the lower Orbs of heaven: but it is now demonstrable unto sense, that there are many more

They were all doomed to be disappointed, however, for the errant engine decided at Stanley junction to spend the remainder of its crowded hour of freedom on the Aberdeen line, and finally came to rest, short of breath, in the dip between Ballathie and Cargill, near the bridge over the Tay.

Roving around; wandering.

Prone to erring or making errors; misbehaving.

We ran down the street in pursuit of the errant dog.

Obsolete form of arrant (“complete; downright, utter”).

Thy company, if I slept not very well / A nights, would make me an errant fool […]

A knight-errant.