'peat-forming' is a compound of the noun 'peat' and the present-participle 'forming'. 'peat' originates from Old English, specifically the word 'pēat', where that root meant 'turf, bog'; 'forming' comes from the verb 'form', ultimately from Latin 'formāre' meaning 'to shape'.

'peat' changed from Old English word 'pēat' (used for turf or bog material) into modern English 'peat'; 'form' came into English via Latin 'formāre' and Old French/Middle English forms and the present participle produced the adjective 'forming', and the two elements combined in modern English to make the compound 'peat-forming'.

Initially, 'peat' primarily referred to 'turf' or 'bog turf', but over time it came to denote the partially decayed plant material (peat) used or studied in ecology. 'peat-forming' thus evolved to mean 'producing or contributing to peat accumulation' in ecological contexts.