Meaning

noun someone who supervises (an examination)

noun someone who gives a warning so that a mistake can be avoided

noun an ironclad vessel built by Federal forces to do battle with the Merrimac

originFrom Latin monitor (“warner”), from perfect passive participle monitus (“warning”), from verb monere (“to warn, admonish, remind”). Warship sense is from USS Monitor, the first ship of this type.

Etymology from Wiktionary, CC BY-SA 3.0