(not) necessarily, (not) entirely, (not) altogether
This structure is used to express that something is not necessarily or entirely the case. It often suggests that a situation is not as bad or as simple as one might have initially thought, or that there is a possibility for a different outcome.
When used with nouns or adjectives, it indicates that the description is not completely true or applicable.
その話は
That story is not necessarily a lie.
彼の意見は
His opinion is not entirely wrong.
It is not necessarily impossible.