'well-thought-out' originates from the combination of 'well,' meaning 'in a good or satisfactory way,' and 'thought-out,' which is the past participle of 'think,' meaning 'to consider or reflect.'

'thought-out' changed from the Old English word 'þencan,' which meant 'to think or consider,' and eventually became the modern English word 'thought.'

Initially, it meant 'carefully considered,' and this meaning has largely remained the same in modern usage.