'well-sourced' is a compound formed from 'well' and the past-participle adjective 'sourced'. 'Well' originates from Old English 'wel' meaning 'well, thoroughly', and 'source' comes from Old French 'sourse' (or 'source'), ultimately from Latin 'surgere' (root 'surg-') meaning 'to rise' (originally referring to a spring or origin).

'well' comes from Old English 'wel' and developed into the modern English 'well'; 'source' passed into English via Old French 'sourse'/'source' (meaning 'spring, origin') and Middle English, and the verb 'to source' (to identify or provide a source) is a more recent English formation; combining 'well' + past participle 'sourced' created 'well-sourced' in modern usage to describe information with good sourcing.

Originally, the elements referred separately to 'thoroughly' ('well') and 'origin/spring' ('source'); over time, 'source' developed into a verb and past-participial adjective, and the compound 'well-sourced' came to mean 'supported by reliable references' rather than anything to do with a physical spring.