Unusually focused in one's attention on a topic, often with great enthusiasm. Frequently in context of an altered state of mind. The opposite of spaced out.
S: How's your Bio studies going?
M: <expounds for several minutes on cellular respiration>
S: ...I see.
M: Sorry, I kinda spaced in.
When a person or situation gets to you so much that all you can think about is that one thing, and one thing only. It's on your mind so much that they might as well settle down in your mind and rent space in you head.
I wont let the results of the job interview Rent Space in my Head