Commentary on Meditations: B8:51

Do not be dilatory in action, muddled in communication, or vague in thought. Don't let your mind settle into depression or elation. Allow some leisure in your life. 'They kill, they cut in pieces, they hunt with curses.' What relevance has this to keeping your mind pure, sane, sober, just? As if a man were to come up to a spring of clear, sweet water and curse it - it would still continue to bubble up water good to drink. He could throw in mud or dung: in no time the spring will break it down, wash it away, and take no colour from it. How then can you secure an everlasting spring and not a cistern? By keeping yourself at all times intent on freedom - and staying kind, simple, and decent.

Dilatory means "slow to act; intended to cause delay."  In today's parlance, Marcus would have said, "don't procrastinate."  When he admonishes himself to not be muddled in communication, he I simply reminding himself to let his talking and writing be clear and concise.  We too should aim for clarity in communication. And when we think, we should think clearly instead of vaguely or imprecisely.

When he admonishes himself to avoid the extremes of depression and elations, he is simply remembering to not let things get him down and not be so overcome with joy and elation that he loses his mind.  So too, we should let our mind and emotions be constant and steady.  The name of the game is: Equanimity. To that end, be sure to allow yourself some down-time and rest.

We can see the back and forth in Marcus’s mind when he rebuts to himself that people kill, cut and hunt with curses. Indeed, people will say mean things about you, however, you must be like the constant spring of clear and clean water.  Try to contaminate it, but it will still flow purely.  

I've heard other phrases similar in concept, such as water on a bird doesn't stick; or sticks and stones may break my bones, but words will never hurt me.  How can you be like a constant, pure spring?  By staying intent on being free in mind, by being kind, simple and a decent person.