A wild olive tree, as opposed to a cultivated olive (kallielaios)
Agrielaios (from agrios, 'wild' + elaios, 'olive tree') refers specifically to the wild olive tree, which produces olives inferior to the cultivated tree. In Romans 11:17, 24, Paul uses this metaphor to describe Gentile believers who have been 'grafted in' to the cultivated olive tree (Israel), a reversal of normal horticultural practice since it is the wild branch grafted to the cultivated root rather than the reverse.
Romans 11:17–24 contains one of Paul's most extended agricultural metaphors. The cultivated olive (Israel) has had some branches removed because of unbelief; the wild olive (Gentiles) has been grafted in contrary to nature. The metaphor makes three crucial theological points: (1) Gentile salvation is a gracious, unnatural act of God — not an entitlement; (2) Jewish roots remain the source of spiritual nourishment for Gentile believers; (3) God can regraft natural branches (Israel) if they do not persist in unbelief. Paul's theology of Jew and Gentile in one body is a mystery revealed in the gospel (Ephesians 3:6).