early 15c., of persons, "to perceive, distinguish;" also, of things, "to refer to, relate to, pertain to," from Old French concerner (15c.) and directly from Medieval Latin concernere "concern, touch, belong to," figurative use of Late Latin concernere "to sift, mix as in a sieve," from assimilated form of Latin com "with, together" (see con-) + cernere "to sift," hence "perceive, comprehend" (from PIE root *krei- "to sieve," thus "discriminate, distinguish").
Apparently the sense of the first element shifted to intensive in Medieval Latin. From late 15c. as "to affect the interest of, be of importance to;" hence the meaning "to worry, disturb, make uneasy or anxious" (17c.). Reflexive use "busy, occupy, engage" ("concern oneself") is from 1630s. Related: Concerned; concerning.
Used imperatively from 1803 (compare similar use of confound); often rendered in dialect as consarn (1832), probably a euphemism for damn (compare concerned). Letter opening to whom it may concern attested by 1740.
1580s, "regard, reference" (a sense now obsolete), from concern (v.). Meaning "that which relates or pertains to one" is from 1670s. Meaning "solicitous regard" is from 1690s. Sense of "an establishment for the transaction of business" is from 1680s.; colloquial sense of "a cumbersome or complicated material object" is from 1824. As nouns in 17c. concernance, concernancy, concernment also were used.