1With reference to a vessel: (sailing) with a strong favourable wind or at full speed; (in later use also) with all sails set.
2figurative Used especially to suggest rapid or unimpeded progress.
3In singular and (occasionally) plural Originally: (the condition of a ship with) sails filled or distended by the wind (chiefly figurative). In later use: (the condition of a ship with) the full complement of sails set.
Under full sail (literal and figurative).
Late Middle English; earliest use found in Morte Arthure. From full + sail.