godan verb (-u)
nearly drown; come close to drowning
Describes a situation where someone almost drowns but survives. The verb 損なう adds the nuance of failing to complete the action, so it literally means 'fail to drown'.
彼は川で溺れ損なったが、大事には至らなかった。
He nearly drowned in the river, but it wasn't serious.
溺れる means 'to drown', while 溺れ損なう means 'to nearly drown' or 'to fail to drown'.
Compound of 溺れ (continuative form of 溺れる 'to drown') + 損なう (auxiliary verb meaning 'to fail to do' or 'to miss doing').