The modal adverb 未必
The modal adverb (语气副词) expresses subjective judgment or assurance toward an action.
The adverb 未必 
is used to express an assumption or uncertainty.
It is placed before a predicate.
is used to express an assumption or uncertainty.
It is placed before a predicate. Pay attention
If a sentence has the adverb 未必, the predicate is often translated into English in the negative form. Whereas in a Chinese sentence, the predicate is not preceded by a negative adverb.
未必 is used more often in written language than in spoken language.
Examples
这个消息未必可靠，咱们再等等吧
This news may be unreliable, let's wait a while
zhè ge xiāoxi wèibì kěkào, zánmen zài děngdeng ba
他未必知道
I don't think he knows
tā wèibì zhīdào
别等了，他未必会来
Don't wait, he is unlikely to come
bié děng le, tā wèibì huì lái