With regard to the MMORP World of Warcraft, dtm+ is an abbreviation for "down to m+?"
m+ is itself an abbreviation for Mythic-Plus dungeons, which are group content events of infinitely increasing difficulty.
One can say "dtm+?" to ask if any of one's guildmates is down for some hardcore group content.
Ãjørn-Khaz'Goroth: dtm+?
Luxio-Khaz'Goroth: yus
Chirran-Dath'Remar: dtf lol
Baptism-Khaz'Goroth: mm
Luvstaspooje-Khaz'Goroth: bonk go to horny jail, ya i'm dtm+ if you sitll have a spot