Inability or unwillingness to express one's emotions. Creates the impression of a cold or unfeeling person. Often associated with having an anal personality. High doses of alcohol are the best known remedy.

(note to editors: I submitted this same definition for "mental constipation", but now I realised that it's really more appropriate for "emotional constipation".)

the inability to express one's feelings despite a sincere effort or desire to do so

That feeling when you just really couldn't give a shit.