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OK so We know Dr. Sloan found out about his nickname when Meredith was on pain killers yelled out woohoo mcsteamy! lol
I do not know or remember how did Derek find out about his nickname Mcdreamy? did anyone tell him?
It was never revealed on-camera that Derek himself found out about his nickname; from the episode it was established, Derek already knew. During "Into You Like a Train" in season 2 when Izzie was on Addie's service, she asks what do 'they' call him, and Izzie tells her 'McDreamy'. Addie responds that must embarrass him, and Izzie replies, "it does".
Actually, Christina nicknames him McDreamy passively to Meredith. It's in Season 1.