weak in something
weak on something

She was not weak in her grasp of the dark.

A market can turn from strong to weak in those two or three weeks.

Still hard to walk very far without getting weak in my legs and have to set down.

Poor Thomas had always been a little weak in the brain.

I took a step back, getting weak in the knees.

But I am not so weak in another way as you think.

The young man must be a bit weak in the head.

It does not show that the economy is weak in any significant way.

I was weak in the middle and could barely keep my eyes open.

The Bush team has been weak in all these areas.