'well-arched' is a compound of 'well' and 'arched'. 'well' originates from Old English 'wel' meaning 'well, thoroughly', while 'arched' ultimately comes from Latin 'arcus' (meaning 'bow') via Old French 'arche' and Middle English 'arche'.

'arched' changed from Latin 'arcus' to Old French 'arche' and Middle English 'arche', becoming modern English 'arch' and its past participle 'arched'; combined with 'well' (from Old English 'wel') to form the compound 'well-arched'.

Initially, 'arch' referred to a 'bow' or curved structural form; over time 'arched' came to mean 'having a curved shape', and the compound 'well-arched' developed the specific sense 'strongly or nicely curved'.