adverb / adjective
away
At a distance from a present or usual place.
The traveler walks away from the cottage along a narrow path.

Etymology
The older expression combined “on” with “way,” picturing someone on the road. Being on one’s way connects departure with being elsewhere or at a distance from a reference point.
In context
ready to be carried below and stowed away in the secret place in which Carne had arranged to hide his treasure.