The Collaborative International Dictionary
Exo-
Exo- \Ex"o-\ [Gr. ? out of, outside, fr. ? out. See Ex-.] A prefix signifying out of, outside; as in exocarp, exogen, exoskeleton.
Douglas Harper's Etymology Dictionary
exo-
word-forming element meaning "outer, outside, outer part" used from mid-19c. in scientific words (such as exoskeleton), from Greek exo "outside," related to ex "out of" (see ex-).
Wiktionary
exo-
pre. outside, external