1. To be or become distracted, preoccupied, or unfocused from the present moment or the task at hand. Thomas, quit spacing out back there and pay attention! I love looking out the window on the train and just spacing out for a few minutes.

2. To be or become confused, disoriented, or stupefied, from or as from drug use. OK, Janet, I know you just took a lot of pills, but you need to stop spacing out for a minute and tell me where Stephen went! In the middle of his speech, he just spaced out and trailed off into an unintelligible murmur.

3. To cause someone to be or become confused, disoriented, or stupefied, from or as from drug use. In this usage, a noun or pronoun is usually used between "space" and "out." Getting knocked in the head like that spaced Tom out pretty bad. I don't like the way these pills space me out—I'd rather just deal with the pain myself.