Etymology
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ruler (n.)

late 14c., "one who rules or governs," especially in reference to superior or sovereign authority, agent noun from rule (v.). The meaning "instrument used for guiding a pen, etc. in making straight lines" is by c. 1400 (compare rule (n.), the older word for this).

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Definitions of ruler

ruler (n.)
a person who rules or commands;
Synonyms: swayer
ruler (n.)
measuring stick consisting of a strip of wood or metal or plastic with a straight edge that is used for drawing straight lines and measuring lengths;
Synonyms: rule
From wordnet.princeton.edu