“A wild olive shoot, grafted in”
Now if some branches have been broken off, and you, a wild olive shoot, have been grafted in among the others to share in the nourishment of the olive root, (Romans 11:17 BSB)
What it sounds like today
A horticulture metaphor, presumably accurate.
The cultural key
Deliberately backwards, and Paul says so. Real grafting runs the other way: cultivated shoots onto wild rootstock. Grafting a wild shoot into a cultivated tree gets you wild fruit; every farmer knew it, and Paul flags his own image as 'contrary to nature' (11:24). (Some ancient agronomists did describe wild grafts to invigorate an old tree, which may lie behind the image, but the 'contrary to nature' admission shows Paul knows the normal direction.) The olive itself is Israel's tree (Jeremiah 11:16), root and branches fed by the patriarchal covenant.
Recovered meaning
Gentile inclusion is agriculturally absurd on purpose: wild wood living off a root it did nothing to cultivate. Which is exactly Paul's warning to Gentile pride in the same breath: 'it is not you who support the root, but the root that supports you.'