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."  Let your talking and writing be clear and concise.  And when you think, think clearly too.  Don't let things get you down and don't be so overcome with joy and elation that you lose your mind.  Let your mind and emotions be constant and steady.  Give yourself some down-time and rest.

People will say mean things about you, but 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 (it doesn't stick); or sticks and stones may break my bones, but words will never hurt me.  How can you be like a constantly, pure spring?  By staying intent on being free (in mind), being kind, simple and a decent person.