I came across this phrase in the first Chapter of The Long Goodbye (written by Raymond Chandler). Is it an idiom? Or something indecent? Here is the context:
"Sure," he said cynically. "Why waste it on a lush? Them curves and all."
"You know him?"
"I heard the dame call him Terry. Otherwise I don't know him from a cow's caboose. But I only been here two weeks."
Best Answer
The standard expression in English is "to not know (someone) from Adam", where Adam is the first man created by God (from the Bible), and it means that you wouldn't recognize that someone if you saw them.
The expression you quote is whimsical, but not indecent. It takes the usual expression a step further: instead of not being able to tell someone apart from another person, you don't even know if they're a person or not.
While "cow's caboose" does refer to the rear end of a cow, the word "caboose" in this sense is one of the least offensive words you could use for "rear end". It's so inoffensive that it's something that you could probably find in children's books (here is an example).