Phlegmatic comes from humoral medicine, which linked excess phlegm with a cool, sluggish, calm temperament. Though it is not usually a diagnosis in modern medicine, the term retains both bodily and psychological meanings. Poets can use it to portray a speaker who responds to crisis with stillness, apparent indifference, or exhausted restraint. It may also color an atmosphere with heaviness, dampness, and inertia. The word is useful because it joins an old medical worldview to character description, allowing physical constitution to mirror mood or moral temperament.

Originating from classical humorism—where an excess of phlegm caused a sluggish, unexcitable temperament—"phlegmatic" now describes someone who is calm, unemotional, cool, or tough to rouse. In character sketches and poetic satires (such as those by Alexander Pope or Geoffrey Chaucer), calling a persona phlegmatic highlights their stolid indifference or stoic reserve, often contrasted against quick-tempered (choleric) or romantic (melancholy) figures. Instructors can use this term to teach characterization and historical personality theories in classic English literature. Related terms include stoic, impassive, placid, and humoral.