To lose sight of something is to stop being able to see it. Lose sight of the boat, lose sight of the point.
The figurative use is the commoner one, since people lose sight of the purpose of a thing far more often than of the thing itself. It happens gradually rather than at once.
1phraselose sight ofTo stop being able to see something.
“They lose sight of the coast about an hour out.”
lose sight of is a phrase.
Practised in Ways of Looking