'phlegm-producing' is a compound formed from the noun 'phlegm' and the present-participle 'producing'. 'phlegm' originates from Greek, specifically the word 'phlegma', where the root meant 'inflammation' or (in humoral theory) the bodily 'phlegm'. 'produce' (root of 'producing') originates from Latin, specifically the word 'producere', where 'pro-' meant 'forward' and 'ducere' meant 'to lead'.

'phlegm' passed into English via Latin 'phlegma' (and Old French forms such as 'flegme'), becoming Middle English 'flegme' and eventually modern English 'phlegm'. 'produce' came from Latin 'producere', through Old French 'produire' into Middle English as 'produce', with the present-participle forming compounds like 'producing'. The compound 'phlegm-producing' is a modern English formation combining these elements.

Initially, 'phlegm' referred to one of the classical bodily humors (inflammation-related mucus) and 'produce' simply meant 'to lead forth' or 'bring about'; over time the compound came to mean specifically 'causing the generation of mucus or phlegm' in a medical or descriptive context.