Douglas Harper's Etymology Dictionary
word-forming element generally meaning "outside, external," before vowels ect-, comb. form of Greek adverb ektos "outside, out of; free from; exempt" (opposed to entos), used to form compounds in Greek (such as ektome "a cutting out"); related to Greek ek, ex "out," from PIE *eghs "out" (see ex-).
The Collaborative International Dictionary
Ecto- \Ec"to-\ See Ect-.
Ect- \Ect-\, Ecto- \Ec"to-\ [Gr. 'ekto`s outside outside.] A combining form signifying without, outside, external.