[Latin -arius or -arium.]
Some (from Latin words in -arius) can refer to a person linked to an activity; examples are secretary (Latin secretarius, a confidential officer), and actuary (Latin actuarius, bookkeeper), a person who uses statistics to calculate insurance risks and premiums. January derives from Januarius, of Janus, the Roman god who presided over doors and beginnings. Other examples are adversary, antiquary, beneficiary, emissary, and notary.
Some nouns in this ending, from Latin words in -arium, have no common theme: aviary (Latin avis, bird); diary (Latin dies, day); and salary (Latin salarium, a Roman soldier's allowance to buy salt, from sal, salt). Others are dictionary, glossary, ovary, sanctuary, and vocabulary.
See also -ery.