so lost in believed that a person cannot understand just what you're doing, what is happening, etc.; preoccupied towards the level of being unaware of your immediate environments.
missing in your mind; abstracted; preoccupied.
(a.) missing at heart; abstracted; preoccupied.
Let the most absent-minded of men be plunged in his deepest reveries--stand that man on his legs, set his feet a-going, and he will infallibly lead you to water, if water there be in all that region.