Green’s Dictionary of Slang

d.d.t.!

1. (US campus) a general excl. of dismissal, contempt [abbr. drop dead! excl. + SE twice].

El Burro (humor mag. of UTEP) Nov. 1/3: He: Darling, is there nothing I can do to make you care? She: D. D. T. He: D. D. T.? She: Yeah — drop dead twice!
[US](con. 1950s) H. Junker ‘The Fifties’ in Eisen Age of Rock 2 (1970) 100: Don’t bug me. Drop dead. DDT.
[US]R. Atkinson Long Gray Line (1990) 14: All of them were also sons of the 1950s [...] and knew that DDT meant ‘drop dead twice’.

2. (US) a command to stop doing something [abbr. don’t do that].

[US]L. Dills CB Slanguage 31: D-D-T: don’t do that.