riddim is no slang, riddim is sacred. this word describes rhythmic patterns that are handed down from generations. virtually all the reggae you are familar with is based on classic riddims, shit even the james bond theme is based on a riddim. riddim is part of the soul it is ancient!
The slight jiggle "in the trunk" that a carribean woman has as she walks down the street. Imagine the 'double beat' sound of reggae, as correlating to a rump rising and falling, jiggling slightly because the woman has some extra fat there.