Please tell me the stem of this word.

This new software has backward compatibility, so you can use old version data without any problems.

He was thinking of a catchphrase to come up with something he wanted to say.

They were having fun chatting at the party.

I had a good time having fun talking with my friends.

She is in charge of watching over the patients at the hospital.

I stayed at home to watch over my sick grandmother.

He was a bit dimwitted, so I had to repeat the explanation several times.

I really think he's a dimwit.

He is considered a dimwit.