Translation:It is not necessarily weak.
"It's not necessarily weak". 弱く（よわく）- Negative form of weak + は Topic particle + ないです - Negative formal ending for adjectives. In this case the particle は makes the negative form less certain, giving not necessarily weak instead of not weak, as the comments above better explain :)
That's actually a quite different meaning and not one expressed in the sentence. "Not necessarily" means it may or may not be, it's equivocal. "not /that/ ..." is a firm statement of lesser degree. Compare. "it's not necessarily illegal"
"its not THAT illegal"
In the first case the speaker isn't telling us if it is or isn't illegal. They suggest that it may not be though.
In the second case the speaker admits it is illegal but thinks its a very minor crime.