annex (v.) Look up annex at Dictionary.com
late 14c., "to connect with," from Old French annexer "to join, attach" (13c.), from Medieval Latin annexare, frequentative of Latin annecetere "to bind to," from ad "to" (see ad-) + nectere "to tie, bind" (see nexus). Usually meaning "to join in a subordinate capacity," but that notion is not in the etymology. Of nations or territories, c. 1400. Related: Annexed; annexing.
annex (n.) Look up annex at Dictionary.com
1540s, "an adjunct, accessory," from French annexe "that which is joined" (13c.), from annexer "to join" (see annex (v.)). Meaning "supplementary building" is from 1861.