As an abundant quantity of water is called ghamr ("ample"), so is a horse devoted to running. As a swift-running watercourse is called ya‘būb ("rushing"), so is a swift-running horse. As a well that won't run dry is called jamūm ("replenished"), so is a horse whose every burst of running is followed by another. As an uninterrupted series of cloudbursts is called al-saḥḥ ("the flow") of rain, a horse is called misaḥḥ ("effluent") if it runs uninterruptedly. If lightness and swiftness are combined in a horse's gait, then it is called fayḍ ("overflowing") and sakb ("outpouring") after the overflowing and outpouring of water.
As the sea's water is inexhaustible, a horse that never tires of running is called a baḥr ("sea"). The Prophet, God's blessings and peace be upon him, was the first to use this expression when he said of his mount: "I find [this horse] to be a baḥr." And so the name Baḥr was given to that horse.
"On descriptors of the horse deriving from descriptors of water":
Section 17.30 of The Statutes of Lexicography by Abu Mansur al-Tha‘alibi
