"Dtj" is an acronym which stands for the phrase "don't tell John." This phrase is typically used when someone has stated something which could prove detrimental if heard by John, who would undoubtedly in turn tell his mother who would then tell the speaker's mother.
Speaker 1: "So, did you and the swiss finally spank the monkey last weekend?"
Speaker 2: "Hellz yea! Dtj, btw."
Speaker 1: "Lolz, poor John..."