Having a stew means to be in a state of excitement, worry, or confusion.
For example he or she got into a stew over nothing.
You could get into excitement over nothing or could be worried about nothing or be confused.
For example you lost your phone or wallet and looked all over the place for it and was worrying about losing your wallet or your phone.
Then you found it later on in the day even though you were really worried you lost it and never would find it.
You got really mad and worried about not being able to find the phone or wallet and so you would have been having a stew.
Just like you could be boiling mad the same goes for having a stew.