To drive over the speed limit, but still more slowly than everyone else on the road.
Underspeed. Conjugate like any other verb. Jeff undersped the whole time we were in Israel, going 80 kph when the posted limit was 60 kph. Krishnan and I mocked him severely when even a bus passed us at about 100 kph.
When you and your friends are passing around a bong, and Y'all smoke the top layer of the weed. The Underweed is all the smokable weed under the burnt top layer in the bowl.
Don't forget to smoke the underweed at the bottom of the bowl!