sentry

Sentry comes from earlier forms sentrie or sentery, and no one is fully sure where it originated. It may be an alteration of sentinel or sanctuary, or it may descend from Old French senteret, meaning 'a path,' itself a diminutive of sentier, from Medieval Latin semitarius.

The facts don't give a clear record of how the meaning shifted over time, only that its roots are uncertain and possibly tangled between two very different ideas: standing guard and walking a path.

If the path theory is right, sentry shares distant roots with sentier, the Old French word for a footpath, by way of Medieval Latin semitarius.