Puppy Dog Eyes
You know, my brother could give you this puppy dog look, and you’d just buy right into it.
– Dean, 1.11 Scarecrow
"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.
In canon the first recorded instance of Sam using this look is in 1.18 Something Wicked, when a young Dean gives the last of the Lucky Charms he was saving for himself to Sam.
The phrase has become a Fanfiction cliche.
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?"