Pool Dude

Papyrus: Nyhe heh heh! I got a swim suit! And it even says cool dude!

Sans: I guess now it says pool dude ;)

Papyrus: SSSSAAAAANNNNSSSS!

Swimsuit
Dude
Skeleton
Funny
Pool
Beach

Explanation

Explain Bear

Okay, so basically, Papyrus got a swimsuit that says "cool dude." Sans, being the funny guy he is, makes a pun and says it now says "pool dude" because it's a swimsuit. It's funny because they are brothers. Also, you probably watch anime. I can tell.