under sail
Etymology
Old English (under, segl)Middle EnglishEnglish nautical terminology
The phrase 'under sail' combines the preposition 'under' (used here in the sense of 'in the state or process of', as in 'under construction' or 'under way') with 'sail' (the fabric surface that catches wind to propel a ship). The expression has been used in English nautical language since the 16th century, when sailing vessels were the primary means of long-distance sea travel.