It's not something that Sam would ever call Castiel. It's not a word that he generally thinks of Castiel in terms of. While Castiel gives himself easily over to Sam, it's a sign of trust.

Though Sam will never, ever get used to the fact of how much Castiel actually wants him.

Or words like that, though he's heard them often enough not to stop when Castiel says it. "I love you, too, angel," he murmurs, twisting his hand slightly so that his fingers glance over Castiel's prostate.