mid-14c., "authority entrusted to someone, delegated authority or power," from Old French commission and directly from Latin commissionem (nominative commissio) "act of committing," in Medieval Latin "delegation of business," noun of action from past participle stem of committere "to unite, connect, combine; to bring together," fromcom "with, together" (see com-) + mittere "to release, let go; send, throw" (see mission).
Meaning "document delegating authority" is from early 15c.; meaning "body of persons charged with authority for the performance of certain special duties" is from late 15c. Sense of "anything entrusted to anyone to perform" is from 1560s; sense of "act of committing or doing" is from 1590s.
Naval sense "period of active service of a warship" is by 1882 (in commission "under the command of an officer" is from 1733). Hence out of commission "laid up in a navy yard or in reserve" (1878), subsequently extended to other machinery, and, figuratively, to persons or human qualities by 1917.
In commercial use, "authority delegated by another for the purchase and sale of goods," 1620s. Meaning "allowance made or percentage given to an agent for transacting business" is from 1725.