Sounds like you missed it, I'm afraid. When you're in the slime forest, you need to save the blue dragon. You'll get the underwear after saving him.
can you show me a screenshot of where i need to save him, please?
It's... really not hard. There's only two paths to take through the forest. You can figure it out I'm sure.
If you're really struggling, I'll say this: if you reached the big green slime puddle, you've gone too far.
