AHH F/UGH posted:

A "dump" or its often-interchangeable but grammatically inaccurate plural "dumps", is shorthand for a "dumper", meaning the buttocks which one "dumps" or poops from. By saying she has "dumps" like a "truck" (commonly used in American English to infer something of large size, such as a car versus a truck), he's alluding that her buttocks is very large, a common braggard staple of African-American community hip hop music as women with large buttocks are often glorified.

Hope that helps, let me know if you have any other questions.