Phlegmatic is the one of the four English words whose sense has hardly shifted. It still means unexcitable, hard to provoke and slow to react, and it still carries a slight edge: to call someone phlegmatic is not quite praise. The older writers were blunter about it. The phlegmatic complexion was described as heavy, pale, soft fleshed, sleepy, dull of apprehension and unwilling to move, and where the sanguine person was too changeable and the choleric too quick, the phlegmatic person was held to be too little of anything.
Astrological medicine assigned the complexion to the Moon, cold and moist in the traditional scheme and associated with everything fluid, changeable and receptive. That assignment softened the portrait somewhat, since the Moon carries associations of sympathy and of imagination that plain sluggishness does not.
Later writers who took the four temperaments as a psychology rather than a physiology have generally rehabilitated it, reading phlegmatic as steady, patient, reliable and difficult to panic, which is much closer to how the word is used in ordinary English now. The rehabilitation is a real change of content and not only a change of tone. A scheme that ranked complexions by how well they served health has been converted into one that treats four types as equally valid, and only the second version can treat being hard to stir as a strength.