The act of moving your body upwards during a period of making out in an effort to obtain fellatio (blowjob) from the partner (can also be used for cunnilingus (eating out), but rarely). Such action usually requires a back and forth (left and right) movement to gain the needed height difference and can be enhanced with a hand to head encouragement motion.
"I tried to slither up on Megan last night but got turned down, 5 times."
"Sliver me timbers!" (used to make fun of those who have attempted at slithering and failed)