Probably from pegnumi (through the idea of gushing plumply); a fount (literally or figuratively), i.e. Source or supply (of water, blood, enjoyment) (not necessarily the original spring) -- fountain, well.
KJV: But whosoever drinketh of the water that I shall give him shall never thirst; but the water that I shall give him shall be in him a well of water springing up into everlasting life.
NASB: but whoever drinks of the water that I will give him shall never thirst; but the water that I will give him will become in him a well of water springing up to eternal life."