Origins
The word 'sail' descends from Old English 'segl' (a sail, a curtain), from Proto-Germanic *seglą (sail). The further etymology is uncertain. One proposal connects it to PIE *sek- (to cut), suggesting the sail was originally conceived as 'a cut piece' of cloth or hide, but this remains speculative. What is certain is that the word is purely Germanic in its attestation and was already well established before the Germanic tribes began their historical migrations.
Cognates span the entire Germanic family: Old Norse 'segl,' Old Frisian 'seil,' Old Saxon 'segal,' Old High German 'segal' (modern German 'Segel'), Dutch 'zeil,' Swedish 'segel,' Danish 'sejl,' Norwegian 'segl.' The phonological development from Old English 'segl' to Modern English 'sail' involved the vocalization of the velar consonant 'g' — the same process that turned 'dæg' into 'day' and 'regn' into 'rain.' The 'g' weakened to a glide and merged with the preceding vowel to produce a diphthong.
The word was borrowed early into several Finnic languages: Old Finnish borrowed a form that eventually gave rise to related nautical terminology, and Estonian preserves similar borrowings. These loans are significant because they testify to direct contact between Germanic-speaking sailors and Finnic-speaking populations around the Baltic Sea well before the Viking Age. The Germanic peoples were already accomplished seafarers in the Bronze and Iron Ages, and their nautical vocabulary left traces wherever they traded.
Old English Period
The verb 'to sail' derives from the noun. Old English had 'siglan' and 'seglian' (to sail), both formed from 'segl' with verbal suffixes. By Middle English, the verb had simplified to 'sailen.' The extension from the literal (to travel by sail) to the figurative (to move smoothly, to glide) appeared by the fourteenth century. Today one can 'sail through' an exam or watch a ball 'sail over' the fence — metaphors built on the effortless gliding of a wind-driven vessel.
As a unit of counting, 'sail' could refer to a single ship: 'a fleet of thirty sail' meant thirty ships. This metonymic usage — naming the whole vessel by its most visible part — was standard in maritime English from the sixteenth through the nineteenth century and survives in literary and historical contexts.
The compound 'sailor' (one who sails) replaced the older 'seaman' as the common term for a mariner during the seventeenth century, though 'seaman' persists in formal and naval usage. Other compounds include 'sailcloth' (the fabric from which sails are made), 'sailboat' (a boat propelled by sails), 'mainsail' (the principal sail on a vessel), and 'foresail' (the sail set forward of the main mast). The spelling distinction between 'sail' (nautical fabric) and 'sale' (an act of selling) was not consistently maintained until the early modern period; both words descend from different Old English roots that happened to converge in pronunciation.