'fused-leaved' is a compound formed from the past participle 'fused' (from the verb 'fuse') and 'leaved' (from 'leaf'). 'Fuse' originates ultimately from Latin, specifically the verb 'fundere' meaning 'to pour', which passed into Old French and late Latin forms and then into English as 'fuse' meaning 'to join by melting or blending'.

'fused' developed as the past participle/adjective form of English 'fuse' and was combined with the adjective-forming element from Old English 'lēaf'/'leaf' (modern 'leaf') to form the descriptive compound 'fused-leaved'.

Initially, 'fuse' carried the sense 'to pour' or 'to melt/join by heat'; over time it broadened to mean 'to join or blend' in general, and in botanical usage this became specialized to describe structures (such as leaves) that are joined together, producing the compound 'fused-leaved'.