Edit: this riddle is intended to be spoken, not read. There are no word clues.
"Wherefore came this plank?" asked Dr. Watson.
"We must first determine the exact whereabouts of it's origin." replied Sherlock Holmes.
"Well, just a moment ago I sat here, minding my manners staring out the window, when I felt drowsy and nearly fell asleep. I awoke quickly as my head began to topple over, feeling as if I was missing something and that is when I saw this flat piece of wood upon the floor where none was before." said Watson, scratching his head.
"What were you doing staring out the window?" inquired Holmes.
"Oh, I really don't recall. I had finished my tea and have no other duties this day therefore I was staring out the window."
"Hmm..." pondered Holmes. "Were you dreaming?"
"I don't recall..."
"Waiting for me!"
"Yes, yes, perhaps..." Watson said flustered. "I don't think I had a plan at all."
"What you have here is a fantastic case of an ineptitude to comprehend modern metaphors. Abire in animo." replied Holmes stoically pointing his pipe at the deceptive lumber. "We must get you a new hobby".
What is Holmes (and this riddle) referring to?