You're debating with your friend over weight loss methods. Your friend says that if you exercise 4 or 5 times per week, you'll definitely lose weight. You think that that's usually true, but you can also remember some examples of people you knew who exercised a lot but didn't lose weight. You say:
That's not necessarily true.
You use the phrase "not necessarily" to show that you don't think something is completely true, although it may be true in most cases or for most of the time.
You use "not necessarily" followed by an adjective:
Food that's labeled as "healthy" is not necessarily any healthier than normal food.
When someone says something you don't think is completely correct, you can also just reply "not necessarily":
A: It's always more expensive to fly on the weekend than on weekdays.
B: Not necessarily.
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