Yours, Etc.: Origins and Uses of 8 Sign-Offs
late 14c., "to connect with," from Old French annexer "to join" (13c.), from Medieval Latin annexare, frequentative of Latin annecetere "to bind to," from ad- "to" (see ad-) + nectere "to tie, bind" (see nexus). Almost always meaning "to join in a subordinate capacity." Of nations or territories, c.1400. Related: Annexed; annexing.
1540s, "an adjunct, accessory," from French annexe, from annexer (see annex (v.)). Meaning "supplementary building" is from 1861.