'phlegmatic' originates from Latin, specifically the word 'phlegmaticus', which in turn comes from Greek 'phlegmatikos' derived from 'phlegma' meaning 'inflammation' or 'phlegm'.

'phlegmaticus' (Late Latin) came from Greek 'phlegmatikos' (φλεγματικός), itself from 'phlegma' (φλεγμα). The word passed into Middle English (via Late Latin/Old French influences) and became modern English 'phlegmatic'.

Initially it referred to being 'of or relating to phlegm' (and, in humoral theory, to a temperament thought caused by phlegm); over time it evolved to mean 'calm, unemotional, not easily excited'.