Impossible to understand, no matter how hard you think about it — so deep or so strange that your mind cannot reach the bottom. A fathom was a measure of depth for water (about 1.8 metres), and to fathom water was to drop a weighted rope and find out how deep it was. So something unfathomable is water where the rope never touches bottom. That picture still lives inside the word, and it is why unfathomable feels bigger than "confusing." You use it for two rather different things. First, for what is genuinely beyond human understanding — the size of the universe, the mind of God, the depth of someone's grief. Here the word is respectful, even awed. Second, and much more commonly today, for a decision so stupid you cannot see how anyone made it: "an unfathomable choice," "for reasons unfathomable to me." Here it is dry and a bit sarcastic. English speakers also use it about people who hide their feelings well — an unfathomable expression, a face you cannot read. Note that it does NOT mean "very deep water" in the plain physical sense; you would not say an unfathomable swimming pool.