tabac stop
A small portion of tobacco placed under the marijuana in a pipe or bong. It's main purpose is to block the hole in the bowl-piece with tobacco so that the weed doesn't fall through. It is also used to add a small head rush to the buzz, and make it burn more smoothly. A good ratio is 95% weed, 5% tobacco.
That first bong crushed me, but the second one had a tabac stop and now I'm so high!