Puppy Dog Eyes
You know, my brother could give you this puppy dog look, and you’d just buy right into it.
"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 fanfiction cliche.
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.
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?"
In 15.10 The Heroes' Journey, the puppy dog eyes no longer work after God has downgraded the Winchesters to normal people. The werewolf Brad comments on Sam's puppy dog eyes and asks if it ever actually works after Sam tries it on him. This is eventually restored when Fortuna bestows the luck of heroes to the Winchesters in 15.11 The Gamblers.