The word εντος appears only twice in the New Testament, in Mt 23:26 and Lk 17:21. Since εντος is related to the widely used preposition εν "in", its meaning can be examined by considering also the usage of εν. Note that the relation between εντος and εν could be compared to that of εκτος "outside, except" and εκ "out of, from".
In Mt 23:26, εντος appears in a reference to the inner surface of a cup. Here, εντος could be seen as an emphatic form of εν. The role of εντος in the phrase το εντος του ποτηριου "the inside of the cup" is not just to indicate something inside of the cup, but something that is inside and belongs to it. This understanding of the word εντος is consistent with all instances in which it is found in the LXX translation of the books of the Old Testament. No LXX passage requires a different understanding of the word. In the LXX, the word εντος is found in Son 3:10, Ps 39:3 (39:4), 103:1, 109:22, Is 16:11, and Dan 10:16. To examine the meaning of εντος, the Hebrew text of these passages and its Greek translation will be considered briefly in each case. Considering first Son 3:9-10, the passage describes a sedan chair of Solomon. In this passage, תוכו רצוף אהבה מבנות ירושלם "its interior inlaid with leather prepared by the daughters of Jerusalem" is translated εντος αυτου λιθοστρωτον αγαπην απο θυγατερων ιερουσαλημ "its interior paved with love from the daughters of Jerusalem". Thus, the word εντος is used in a reference to the surface of the interior of the sedan, which clearly was something that not only was inside of the sedan but also a part of it. Considering also Ps 39:3 (39:4), εθερμανθη η καρδια μου εντος μου "my heart was hot within me" translates חם לבי בקרבי "my heart was hot within me". Since the word εντος refers to the heart, it is used to describe something that is inside of the man and is part of him. The same comment could be made about Ps 109:22, in which και η καρδια μου τεταρακται εντος μου "and my heart is troubled within me" translates ולבי חלל בקרבי "and my heart is pierced within me". In Ps 103:1, ברכי נפשי את יהוה וכל קרבי את שם קדשו "Bless the LORD, O my soul, and all that is within me bless his holy name" is translated ευλογει η ψυχη μου τον κυριον και παντα τα εντος μου το ονομα το αγιον αυτου "Bless the Lord, O my soul, and all that is within me bless his holy name". The passage urges the hearer to involve his whole inner being when praising the Lord. Note that "all that is within me" (כל קרבי and παντα τα εντος μου) corresponds to "my soul" (נפשי and η ψυχη μου). Therefore, it seems quite unlikely that "all that is within me" includes also what is within the man that does not belong to the soul (such as an unclean spirit). It follows that once again, εντος refers to something that is inside of a man and belongs to him. In Dan 10:16, נהפכו צירי עלי ולא עצרתי כח "my pains have come upon me, and I have retained no strength" is rendered εστραφη τα εντος μου εν εμοι και ουκ εσχον ισχυν "my inward parts were turned inside of me, and I had no strength" in the Theodotian version. Understanding the passage to speak about the condition of the inner being, the word εντος continues to refer to what is inside of man and belongs to him. In Is 16:11, מעי למואב ככנור יהמו וקרבי לקיר חרש "my inward parts groan like a harp, and my inward being for Kir Hareseth" is rendered η κοιλια μου επι μωαβ ως κιθαρα ηχησει και τα εντος μου ωσει τειχος ο ενεκαινισας "my inner being will sound like a harp for Moab, and my inward parts are like a wall that thou hast repaired". Since the passage associates η κοιλια μου "my inner being" with τα εντος μου, it follows that τα εντος μου should be understood as referring to that which is inside the inner being and belongs to it. (As for translating κοιλια with "inner being", see Jn 7:38.)
In Lk 17:21, the word εντος appears in the phrase εντος υμων. If εν were used instead, the phrase would have been εν υμιν instead of εντος υμων. The meaning of εν υμιν could be "inside of you", such as in Mt 10:20, Ac 13:15, or "among you", such as in Lk 10:13, Lk 22:26. Since the plural form of the pronoun is used in εντος υμων, it could be concluded that its meaning must be close to the meanings of εν υμιν "inside of you, within you" and "among you". However, since Lk 17:21 uses the rare phrase εντος υμων instead of the common εν υμιν, there must be a difference in meaning. In view of the remarks above, the meaning of εντος υμων must be "that which is your own, both that which is in you and that which is among you". Thus, in its context, Lk 17:21 indicates that when the kingdom of God will come, even those things that people consider their own will belong to the kingdom. See the article On Lk 17:20-37.