An assistant on any locomotive, whether steam-powered or not.
A safety inspector in coal mines.
Someone (especially one who is male) who is skilled in the work of fighting fire.
A relief pitcher (reflecting the figurative analogy of rescuing the situation).
A person (originally a man) who keeps the fire going underneath a steam boiler (originally, shoveling coal by hand), particularly on a railroad locomotive or steamship.
A sentry, watch, or guard.
A unique string of characters recognised by a computer program for processing in a special way; a keyword.
A sentinel crab.
A sign of a health risk (e.g. a disease, an adverse effect).
To post as guard.
To post a guard for.
To watch over as a guard.