late 14c., "to belong to as a possession or right," from Old French apendre (13c.) belong, be dependent (on); attach (oneself) to; hang, hang up," and directly from Latin appendere "to cause to hang (from something), weigh," from ad- "to" (see ad-) + pendere "hang" (see pendant).
Meaning "to hang on, attach as a pendant" is 1640s; that of "attach as an appendix" is recorded by 1843. OED says the original word was obsolete by c.1500, and these later transitive senses represent a reborrowing from Latin or French. Related: Appended; appending.