late 14c., "to join or unite (something to something else)," from Latin addere "add to, join, attach, place upon," literal and figurative, from ad "to" (see ad-) + -dere, combining form meaning "to put, place," from dare "to give" (from PIE root *do- "to give").
The intransitive meaning "to do sums, do addition" also is from late 14c. Related: Added; adding. To add up is from 1754; in the figurative meaning "make sense," by 1942. Adding machine "machine to cast up large sums" is from 1822.
Old English togædere "so as to be present in one place, in a group, in an accumulated mass," from to (see to) + gædere "together" (adv.), apparently a variant of the adverb geador "together," from Proto-Germanic *gaduri- "in a body," from PIE *ghedh- "to unite, join, fit" (see good, and compare gather).
In reference to single things, "so as to be unified or integrated," from c. 1300. Adjective meaning "self-assured, free of emotional difficulties" is first recorded 1966. German cognate zusammen has as second element the Old High German verbal cognate of English same (Old English also had tosamne "together").
1520s, "to give or grant in common with others," from Latin contributus, past participle of contribuere "to bring together, add, unite, collect, contribute" from assimilated form of com "with, together" (see con-) + tribuere "to allot, pay" (see tribute). Figurative sense is from 1630s. Related: Contributed; contributing.
c. 1400, from Latin aggregatus "associated, united," past participle of aggregare "add to (a flock), lead to a flock, bring together (in a flock)," figuratively "attach, join, include; collect, bring together," from ad "to" (see ad-) + gregare "to collect into a flock, gather," from grex (genitive gregis) "a flock" (from PIE root *ger- "to gather").
c. 1400, "anything added, an increase or increment," from Latin additamentum "an increase," from past-participle stem of addere "to add" (see add).
late 15c., "play or sing in harmony," from French harmoniser (15c.), from Old French harmonie (see harmony). Meaning "be in harmony (with), go well together" is from 1620s. Transitive sense "bring into harmony" is from 1700; figurative sense "bring into agreement" is from 1767. Meaning "add harmony to (a melody)" is from 1790. Related: Harmonized; harmonizing.