the GNU version of the Collaborative International Dictionary of English
ad. In a dense, compact manner.
The Century Dictionary and Cyclopedia
In a dense manner; compactly.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
ad. in a concentrated manner
ad. in a stupid manner
Word Usage
"Following your logic, no one would ever go after terrorists hiding in densely populated areas, and terrorists would use that immunity to continue attacking from such areas."