Why is the ninth month called September?
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.