As of when a girl is giving a blowjob, and stops to take a breath, but faces the dreaded strings of saliva/"pre-ejaculation fluid" that remain stuck between her mouth and the penis she is sucking.
So called because it resembles a spider web with morning dew on it.
"I was enjoying being sucked off so much last night I started to ooze. Sadly this was at the same time as the missus was taking a breath and we got spider juice. Legend has it that the Joneses down the road bottle and sell the stuff."