One who hallows or makes something holy; a person who sanctifies or treats something as sacred.
From 'hallow' (to make holy) with the -er agent noun suffix, creating a noun for one who performs the action of hallowing.
The -er suffix creates 'doers'—'teacher,' 'worker,' 'hallower'—and it's so automatic that English speakers invented entirely new words like 'googler' as soon as Google became a verb.
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