Merlin was spending a lot of time thinking about Arthur's hands.
Strong hands. Rough in spots, from being a fighter and a hunter. The rest soft and milky. Smooth.
Merlin thought about his mouth on Arthur's hands, too. Gently kissing the knuckles, or the patch of skin hardened from wielding the handle of his sword. Or licking the fingers clean when Arthur came in from a hunt, dirt and sweat and who knows what else smeared on his fingers.
When Merlin draws a bath for him, he tells Arthur, casually as he can, "Test the water, see it's not still too hot." And Arthur's fingers dip into the tub, run a line just below the surface, and for just a moment, Merlin imagines he is water, imagines Arthur's hands gliding across his body, testing for heat and readiness.
And when Merlin lay alone in his bed, thinking of Arthur, he imagined Arthur behind him, pushing into him, whispering that he must be quiet so that the castle cannot hear. And in this fantasy, Arthur clamps his hand on Merlin's mouth to command his silence, and as Merlin's lips slightly part, Arthur's ring finger slips inside his mouth, moves in rhythm to the thrusts below.
But Merlin doesn't live in a world of fantasy. In reality, he is just Arthur's servant.
And tonight, Arthur is finicky with his food. He is pushing the meat and vegetables around his plate with his finger, as if the motion will make them more appearing, or perhaps make them disappear. Merlin is staring at his hands, as the sheen of grease on them, at their bored and nimble play.
Arthur catches him tonight. He knows Merlin is staring.
He says, "For goodness sakes, Merlin, if you want my tablescraps that badly, take them," and gets up from the table to tend to something more important than letting Merlin watch him eat.
Merlin smiles, insincere or small, and pops a piece of gamebird into his mouth. He lets the assumption stand.
It's hardly the only secret he keeps from Arthur, and maybe not the worst.