Puppy Dog Eyes
From Super-wiki
"Puppy Dog Eyes" is the term used by fans to describe the endearing, pleading look Sam gives Dean when he wants something. Dean usually finds this tactic irresistible.
The phrase has become a Category:Fanfiction cliche.
see also Bitch Face
In 5.09 The Real Ghostbusters when Dean and Sam are trying to convince a girl to act as bait for the ghosts, Dean says to Sam "Just give her the puppy dog thing, okay?"