From Old English 'hǣlan' (to make whole), from Proto-Germanic *hailaz — the root behind health, whole, and holy.
To make or become sound or healthy again.
From Old English 'hǣlan' (to make whole), from Proto-Germanic *hailjaną, from *hailaz (whole, healthy). Same root gives 'health,' 'whole,' and 'holy.' Key roots: *hailaz (Proto-Germanic: "whole, healthy").
'Heal,' 'health,' 'whole,' and 'holy' all come from the same root meaning 'complete.' To heal is to be made whole.