Basically, when someone does not have a feeling (specifically, anger), but due to the constant prodding of other characters develops that feeling. Quote above is paraphrased from an old (90's) Drabble comic. Anger is the most common end result due to Rule of Drama, but any emotion reached after a steady (and in the moment true) insistence that it is not present counts.
Contrast That Makes Me Feel [...] [tvtropes.org]