'narrow-leafed' originates from Modern English as a compound formed from the adjective 'narrow' and the noun 'leaf' with the adjectival/-ed suffix '-ed'.

'narrow' changed from Old English 'nearwe' through Middle English 'narwe' to the modern English 'narrow', and 'leaf' comes from Old English 'lēaf' which became Middle English 'leaf' and then modern 'leaf'; the compound form developed in Modern English by combining these elements with '-ed'.

Initially the components meant 'not wide' (for 'narrow') and 'leaf' (for 'leaf'); over time they were combined in compounds to mean 'having narrow leaves', which is the current usage.