When the baggage (i.e. commitment issues) from a previous relationship causes a person to break up with their new partner, and as a result, that new partner gains the baggage of the dumper, which causes problems in the new partners future relationships. Similar to an S.T.D., except mental rather than physical.
"Dexter, why don't you just date that cute girl from school that texts you all the time?" "I can't! Catherine gave me R.T.B. (Relationship Transmitted Baggage), and ever since I have serious commitment issues."