'tranquil-eyed' originates from the English word 'tranquil,' which comes from the Latin word 'tranquillus,' meaning 'calm, quiet.' The suffix '-eyed' is used to describe the appearance of the eyes.

'tranquillus' transformed into the Old French word 'tranquille,' and eventually became the modern English word 'tranquil.' The combination with '-eyed' is a more recent English formation.

Initially, 'tranquil' meant 'calm or quiet,' and this meaning has largely remained the same in modern usage when combined with '-eyed.'