"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..."
acronym stainding for "Don't tell Jen". Used most often in the context of a culinary-oriented conversation, it indicates the desire to not divulge any information regarding a food item to Jen, lest she appropriates and devours it.
hey Tom, I got these incredible cookies from home the other day. DTJ ok?