'well-selected' originates from English as a compound of the adverb 'well' and the past participle 'selected'. 'Well' comes from Old English 'wel' meaning 'in a good or proper way', and 'select' ultimately comes from Latin 'seligere' (past participle 'selectus'), where 'se-' meant 'apart' and 'legere' meant 'to choose or pick'.

'select' changed from Latin 'seligere' / 'selectus' into Late Latin and Old French forms ('select'), then entered Middle English as 'select' and later formed past participle 'selected'; combined with the adverb 'well' it produced the compound adjective 'well-selected' in modern English.

Initially it literally meant 'chosen apart in a good way' (i.e., 'chosen carefully'), and over time this straightforward meaning has largely remained the same as the modern sense 'carefully chosen'.