knees went watery

[Vanloon]

In the book 'On my honor' by Marion Dane Bauer
p 8,

Joel's father shook his head. "I doubt that, but I guess it wont's hurt you boys to be good and tired tonight."
Joel's knees went watery. His father was going to say they could go!

What dose that mean?
Thanks in advance!

 

[owlman5]

Hi, Vanloon.

It means that Joel was so excited that he couldn't stand normally. His legs were temporarily weakened by excitement.

 

[Vanloon]

Thank you Mr Owlman5!

I want to know the origin of those phase if possible.
On the dictionary, I cannot find out.
Thanks!!

 

[owlman5]

You're welcome. I don't know its origin, Vanloon. I haven't heard "watery" used with "knees" before, but it surely means the same thing as "weak" or "wobbly", which are common: Joel's knees went weak/wobbly when he found out that he could go.

 

[Vanloon]

Thank you Mr. Owlma5! You are so kind!!