mrdr is an acronym for "Message Removed; didn't read". It is primarily used in chats that allow edit or removal of text. Commonly used for people who don't check their Skype at work fast enough for juicy gossip that needs to be removed so other don't see it or quote it later.

Example, skype conversation at the office
Co-worker 1: Totally just caught Cindy and David in the back office... 11:45 am
*Co-worker 1 removes message*

Co-worker 1: This message has been removed. 11:45am
Co-worker 2: ???
Co-worker 2: mrdr
Co-worker 2: You removed it too fast!!! What did you say!?

Noun: a friend who is usually only called upon by a friend when a more primary/prominent friend is unavailable.

Taken from the term “second string” in an athletic competition situation. In football, if the star quarterback gets injured during a play, the second string is called off the bench to replace him/her. A “Second String Friend” is essentially benched until needed, if ever.

Synonyms: Plan B, Second Choice, Secondary Friend, Benched Until Needed