1. Indulgence in aimless thought or dreamy imagining; absent-mindedness.
2. gathering of the tufts of wool shed by sheep and caught on bushes.
Really, God thought with annoyance, this woolgathering —at such a moment! (Damon Francis Knight, The Worshippers)
His efforts to explain his success keep wandering off into sunny vales of academic woolgathering (Washington Post, 1998)
Woolgathering once literally referred to the act of gathering loose tufts of wool that had gotten caught on bushes and fences as sheep passed by. Woolgatherers must have seemed to wander aimlessly, gaining little for their efforts, for in the mid-16th century "woolgathering" began to appear in figurative phrases such as "my wits (or my mind) went a-woolgathering" - in other words, "my mind went wandering aimlessly." From there, it wasn't long before the word woolgathering came to suggest the act of indulging in purposeless mind-wandering. (Merriam Webster)