A high possibility. One can infer that something that is described as a 'deep poss' has a good chance of happening. The depth of the poss can be further described to more precisely detail the chance that something is going to happen. Many senses can be involved here and generally more comforting descriptions elucidate a deeper poss. A deep warm poss, for example, is comforting and therefore likely to occur.
ANTONYM: Shallow poss
Person 1 - "Bonkwoggling tonight anyone?"
Person 2 - "That is a deep poss"