d.t.r., phr. — Green’s Dictionary of Slang

d.t.r. phr.

[abbr.; used of a conversation, often between the partners]

(US campus) defining the relationship.

1996		Eble Sl. and Sociability 36: Two kinds of shortening reduce words to letters. The dominant type in college slang is initialism, which names the individual letters: [...] DTR (defining the relationship).
2003		Eble Campus Sl. Nov.
2012		C. Eble (ed.) UNC-CH Campus Sl. 2011.