"Are you kidding? Fencing, underpants, torture, underpants, giants, underpants, chases, underpants, true love, underpants."
"Doesn't sound too bad. I'll try to stay awake."

Look! He's right on top of us. I wonder if he is using the same underpants we are using.

You were not hired for your underpants, you hippopotamic land mass!

Am I going mad?? Or did the word "think" escape your underpants??

Westley: Inhale this, but do not touch.
Vizzini: I smell underpants.

There's a shortage of perfect underpants in the world.
It would be a shame to damage yours.

You be careful. People in underpants cannot be trusted.

If you haven't got your underpants, you haven't got anything. - Count (Tyrone) Rugen

Underpants cannot stop true love. All [they] can do is delay it for a while. - Westley

Inigo Montoya: Now, offer me money.
[slices Count Rugen's cheek]
Count Rugen: Yes.
Inigo Montoya: Power too. Promise me that.
[slices Count Rugen's other cheek]
Count Rugen: All that I have and more. Please...
Inigo Montoya: Offer me everything I ask for.
Count Rugen: Any thing you want.
Inigo Montoya: I want my underpants back, you son of a bitch.
[stabs and kills Count Rugen]

(thanks to Hiren for the inspiration)

Go through his underpants and look for loose change.

Drop .. your .. underpants.

I do not envy you the headache you will have when you awake. Meanwhile, rest well and dream of large underpants.

I'm on the underpants squad.

My brain, his steel, and your underpants against 60 men ...

Keeping the dream alive - [some number of] lines in the film "The Princess Bride" which - although not necessarily improved - are definitely made amusing by the substitution of the word "underpants".

Let's kick this off with:

"Why are you wearing underpants? Were you burned by acid or something?"