irregular in performance, behaviour, or attitude; inconsistent and unpredictable
2.
having no fixed or regular course; wandering
—n
3.
a piece of rock that differs in composition, shape, etc, from the rock surrounding it, having been transported from its place of origin, esp by glacial action
4.
an erratic person or thing
[C14: from Latin errāticus, from errāre to wander, err]
late 14c., from O.Fr. erratiquem from L. erraticus, from erratum, pp. of errare "to wander, err" (see err). Sense of "irregular, eccentric" is 1816. Related: Erratically.