Having your mind focused on something else so much that you're not paying attention to what's happening around you.
From 'pre-' meaning before, plus 'occupied' (from Latin 'occupare' meaning to seize or take up). The prefix suggests your mind is taken up ahead of time by something else. The word emphasizes that your mental space is already claimed.
Your brain has limited 'attention bandwidth'—scientists call it your working memory—and once it's filled with worry, a song, or a problem, there's literally no room left to focus on the present moment. This is why preoccupied people seem absent-minded; they're not being rude, they've just physically run out of brain space for the current moment.
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