Standing for "Deep and Meaningful" standing for when two (or more) friends are at a pub, club or party aside from other friends talking privately about something major (or seemingly Major). It can be a couple in a relationship talk
but can just as likely be friends talking about earlier events or comments that have affected them, and wish to discuss in private without leaving the pub/party/club/etc.
With Circles of friends, when they are in a D+M it is advised to steer clear of them until it is finished unless you are definitely sure you're sure of the topic and it geneuinely relates to you.
If unsure ALWAYS check with someone else within the group of friends, who is NOT participating in the D+M to check whether it is ok to approach them and talk.