From Margaret R. Graver's Stoicism and Emotion:
"The causes of emotion, then, are exactly like the causes of other sorts of impulse. But one does not therefore have to say that all impulses are alike. Introspection insists that anger, fear, grief, and desire have a "hot" or vehement quality to them that sets them apart from run-of-the-mill actions like walking or brushing one's teeth. It is in this that emotions seem to run roughshod over our better judgment, compromising our sense of agency and self-management. That special feature is picked out by the early Stoic terminology of pleonasmos or `excessiveness.' Already in the writings of Zeno of Citium, an emotion is defined as an excessive impulse, a hormē pleonazousa. Or it maybe characterized as `disobedient to reason' or `turned away from reason'; these expressions, too, are probably Zenonian in origin.
Our word "excessive" is not an entirely satisfactory translation for Zeno's term pleonazousa. Built on a root meaning `more' (pleon), the Greek word is in its grammatical form a present participle of the verb pleonazō, meaning `be more' or `exceed' or `go beyond bounds.' Hence it is `exceeding' rather than `excessive.' The difference of inflection could easily be felt by a Greek speaker. The unnamed Stoic source quoted by Stobaeus comments on Zeno's preference for the verb over the related adjective pleonastikē.
'He [Zeno] does not say an impulse whose nature it is to exceed,' but `one that is in fact exceeding.' For it is not a matter of the capacity but of the activity. '
This interprets Zeno to have said that the excessiveness which characterizes emotional impulses is not some inherent attribute belonging to them by nature. It is just what such impulses regularly do in human beings as we know them: they exceed something. What they exceed, and how, is not stated here, but an answer is ready to hand: they exceed other impulses which the same agents might form, and sometimes do form, in accordance with correct reason."
I find this image of emotion as an excessive overflow of impulse to be a compelling one. The idea that it wells up like overflowing water and drowns out reason unless reason is exercised to stop its flow seems accurate in many ways - assenting to a proposition that an emotion should affect us is equivalent to opening a valve and allowing the water to flow. Reason can be exercised to withdraw assent and shut of the valve.
Margaret R. Graver. Stoicism and Emotion (pp. 66-67). Kindle Edition.