"piece of shaped cloth spread so as to catch the wind and cause a vessel to move in water," Old English segl "sail, veil, curtain," from Proto-Germanic *seglom (source also of Old Saxon, Swedish segel, Old Norse segl, Old Frisian seil, Dutch zeil, Old High German segal, German Segel), of obscure origin with no known cognates outside Germanic (Irish seol, Welsh hwyl "sail" are Germanic loan-words). In some sources (Klein, OED) referred to PIE root *sek- "to cut," as if meaning "a cut piece of cloth."
As "a single ship or vessel" by 1510s. To take the wind out of (someone's) sails (1888) is to deprive (someone) of the means of progress, especially by sudden and unexpected action, "as by one vessel sailing between the wind and another vessel" ["The Encyclopaedic Dictionary," 1888].
Old English segilan "travel on water in a ship by the action of wind upon sails; equip with a sail," from the same Germanic source as sail (n.); cognate with Old Norse sigla, Middle Dutch seghelen, Dutch zeilen, Middle Low German segelen, German segeln. Later extended to travel over water by steam power or other mechanical agency. The meaning "to set out on a sea voyage, leave port" is from c. 1200. Extended sense of "float through the air; move forward impressively" is by late 14c., as is the sense of "sail over or upon." Related: Sailed; sailing.