Be absentminded or impractical, as in She must have had her head in the clouds when she made the reservations, because they never heard of us , or He'll never be able to run the business-he's always got his head in the clouds. This idiom uses in the clouds in the sense of "fanciful" or "unreal," a usage dating from the mid-1600s.
See also: have, head, in, the
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.