"not in the weak"

Have you some context?

I've never heard this before, and as it stands, I would say it is ungrammatical and meaningless.

There are many possible contexts: mathematics, politics, particle physics...

eg

Strangeness and charm are conserved in the strong and electromagnetic interactions but not in the weak interactions.

It may mean "not in weak people" (e.g. perhaps some trait is not possessed by weak people, or something like that). However, without context it is impossible to be sure.