To Drop Thun Thun Thun is American Slang for taking ecstasy. A Thun Thun Thun is a small pill that comes in the form of ecstacy. To “drop” something can be used as slang for swallowing something.
Dropping Thun Thun Thun is slang that is rumored to have origniated in the Bay Area. Thun, Thun Thun, and Thun Thun Thun all seem to be acceptable ways to use this word for ecstacy.
Below are the lyrics to the song by the Finatticz called “Don’t Drop that Thun Thun” which has greatly increased the popularity of this slang term.