To adorn unnecessarily something already beautiful. The expression is a condensation of Shakespeare's metaphor in King John: "To gild refined gold, to paint the lily ... is wasteful and ridiculous excess." i.e., you wouldn't need to add gold to a beautiful lily.
Gilding the lily meaning...
To disguise, to adorn, to camouflage, to take the piss, to deceive, to make something look bigger than what it really is, exaggeration, to embellish, telling lies, telling tales. The list goes on.
When a horny, wet female climbs on top of the male in cowgirl position and glides his rigid penis back and forth between her engorged pussy lips, at a vigorous pace, until he busts his nut. Gliding does not involve vaginal penetration. And requires extreme vaginal wetness for the maximum glide.
Rachel did a little cowgirl gliding on me last night. She was going so fast I blew nut in like2 minutes.