confound (v.)

early 14c., "discomfited, routed, defeated" (of groups), serving at first as an alternative past participle of confound, as Latin confusus was the past participle of confundere "to pour together, mix, mingle; to join together;" hence, figuratively, "to throw into disorder; to trouble, disturb, upset."

The Latin past participle also was used as an adjective, with reference to mental states, "troubled, embarrassed," and this passed into Old French as confus "dejected, downcast, undone, defeated, discomfited in mind or feeling," which passed to Middle English as confus (14c.; for example Chaucer's "I am so confus, that I may not seye"), which then was assimilated to the English past-participle pattern by addition of -ed. By mid-16c., the word evolved a back-formed verb in confuse. Few English etymologies are more confusing. 

Of individuals, "discomfited in mind, perplexed," from mid-14c.; in logic, "indistinct, indistinguishable from other ideas from which it ought to be different," 1610s. Meaning "lacking orderly arrangement of parts" is from 1776. Related: Confusedly.

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.