Synonym of fourth base.
More specifically, when two people go to the fourth floor on an elevator. The power goes out on the way and the elevator falls. Thinking it's their last moments of life and wanting to lose their virginity before they die, they have sex. The power comes back on and they reach the fourth floor. Afterwards it is always awkward between them.

Dejagoo is the strange feeling that you've been in this sticky situation before. It is dejavu due to goo. Usual symptoms are shivers down once's spine followed by disgust, especially if actual goo is involved.