means A wind blowing in the same direction you're traveling, giving you a helpful boost; by extension, any favorable force that speeds your progress.

from A plain compound of "tail" and "wind" — the wind coming from behind, at your tail end. "Tail" traces back to Old English "tægl," the rear appendage of a beast, while "wind" comes from Old English "wind," cousin to Latin "ventus." The term took flight (literally) in the age of sailing and later aviation, where a wind at your back meant faster passage and saved fuel — its opposite, the dreaded "headwind," was named on the same logic.