due (adj.)

mid-14c., "customary, regular, right, proper;" late 14c., "owed, payable as an obligation, owing by right of circumstance or condition," from Old French deu, past participle of devoir "to owe," from Latin debere "to owe," originally, "keep something away from someone," from de- "away" (see de-) + habere "to have" (from PIE root *ghabh- "to give or receive").

Of actions, "conscientious, careful," late 14c. Meaning "that is to be expected or looked for" is by 1833. Phrase in due time "at a set time; at an appropriate time" is from late 14c. Due to is from early 15c. as "deserved by, merited by;" also "owing to." It is attested from 1660s as "attributable to as a cause or origin." Its use as a prepositional phrase (much maligned by grammarians) is by 1897.

due (n.)

early 15c., "that which is owed, what one deserves or is entitled to," from due (adj.), also compare dues. To give the devil his due "do justice to a person of supposed bad character" is from 1590s.  "Giue them their due though they were diuels" [1589]. 

due (adv.)

1590s, "duly," from due (adj.). In reference to points of the compass, "directly, exactly" (as in due east) it is attested from c. 1600, originally nautical, from notion of "fitting, rightful."

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Definitions of due from WordNet
due (adj.)
scheduled to arrive;
the train is due in 15 minutes
due (adj.)
owed and payable immediately or on demand;
payment is due
due (adj.)
suitable to or expected in the circumstances;
in due course
due cause to honor them
exercising due care
all due respect
due esteem
a long due promotion
due (adj.)
capable of being assigned or credited to;
the cancellation of the concert was due to the rain
Synonyms: ascribable / imputable / referable
due (n.)
that which is deserved or owed;
give the devil his due
due (n.)
a payment that is due (e.g., as the price of membership);
the society dropped him for non-payment of dues
due (adv.)
directly or exactly; straight;
went due North