'well-tended' is a compound of the adverb 'well' + the past participle 'tended'. 'tend' originates from Latin, specifically the word 'tendere', where 'tendere' meant 'to stretch or extend'; it entered English via Old French and Middle English.

'tendere' passed into Old French as 'tendre' (sense related to stretching/aiming), then into Middle English as 'tenden'/'tenden' with senses like 'to direct' or 'to attend to', eventually becoming modern English 'tend'. 'well' comes from Old English 'wel' and continued as the adverb 'well' in Modern English. The compound 'well-tended' formed in Modern English by combining 'well' + 'tended'.

Initially 'tendere' meant 'to stretch/extend'; over time 'tend' shifted toward senses of attending to or caring for something, and 'well-tended' came to mean 'kept or cared for in good condition'.