Knowing someone you're interested in doesn't feel the same way so you feel rejected without ever having made an advance. Deja-rejection usually occurs in males with low self-esteem and results in feelings of embitterment directed towards the object of affection and sour grapes over a rejection that never occured.
Ex.: Kyle just got deja-rejected by that chick in his neurobiology class... Guess that's gonna make things pretty awkward for them.
Ex.: Dude, you're letting yourself sink into deja-rejection with every girl you meet since your breakup. You need to start liking yourself so you can like these hot bimbos again.