The modern usage is dismissive, often suggesting an unimportant or trivial context, and this sense was beginning to be used in Shakespeare's time, as in Jaques' famous line "All the world's a stage, And all the men and women merely players" (As You Like It, II.vii.141). But there was an earlier sense, which died out in the 18th century, and this has a much stronger meaning of "utterly, entirely". To miss the strength of feeling can result in a seriously misleading interpretation. When Rosalind, in the same play (III.ii.383), describes love as "merely a madness", she is not playing it down: on the contrary. And when Hamlet compares the world to an unweeded garden, saying that "Things rank and gross in nature Possess it merely" (Hamlet, I.ii.137), he means that the weeds are everywhere. The nuance needs the actor to use a positive tone of voice.
David Crystal is author, with Ben Crystal, of Shakespeare's Words, published by Penguin