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1620s (also in early use as an adjective), from Church Latin clericus "clergyman, priest," noun use of adjective meaning "priestly, belonging to the clerus;" from Ecclesiastical Greek klerikos "pertaining to an inheritance," but in Greek Christian jargon by 2c., "of the clergy, belonging to the clergy," as opposed to the laity; from kleros "a lot, allotment; piece of land; heritage, inheritance," originally "a shard or wood chip used in casting lots," related to klan "to break" (see clastic).
Kleros was used by early Greek Christians for matters relating to ministry, based on Deut. xviii:2 reference to Levites as temple assistants: "Therefore shall they have no inheritance among their brethren: the Lord is their inheritance," kleros being used as a translation of Hebrew nahalah "inheritance, lot." Or else it is from the use of the word in Acts i:17. A word taken up in English after clerk (n.) shifted to its modern meaning.