As it sounds, a very uncomfortable, while also very awkward, situation or scenario. Both are similar in use, but this term is used when the situation begs for a word stronger than either of the 2.
Typically, an uncomfortable situation would be walking into your home to find your roommate making out with his/her girlfriend/boyfriend. The situation would then be awkward if your roommate was making out with his/her girlfriend/boyfriend - lacking some clothing and catching glimpse of things you did not want to see.
Now imagine walking in on your roomate, lacking clothes and making out with YOUR boyfriend/girlfriend. That's UNCOMFAWKWARD!