Inter canem et lupum is an archaic Latin proverb referring to twilight; literally, "between the dog and the wolf". The thought is that dogs go to sleep as the day comes to an end, and wolves come out to hunt after sundown. This was applied metaphorically to humans as well; good people were safe in their homes after sundown, and creeping wolfishly through the streets at night was evidence of ill intent.
An alternative, and more modern definition, may be derived from this; some will say that inter canem et lupum is the time when light is too poor to tell a dog from a wolf. This may be a confabulation between the poetical use of this phrase in legal documents with an old definition of twilight as the time when it becomes too dark to "discern the countenance of a person" -- that is, discern a friend from a thief. This was legally relevant because the crime of breaking and entering became the more serious crime of burglary inter canem et lupum.
While this proverb has not made the jump into English, it is just common enough to have picked up a third meaning, and is sometimes used to mean 'between a rock and a hard place'. This is clearly and unambiguously an incorrect usage, and should be stamped out wherever it is spotted.
Brevity Quest 2016