Is it ensure, insure, or assure?
early 13c., from Old Norse drupa "to drop, sink, hang (the head)," from Proto-Germanic *drup-, from PIE *dhreu-, related to Old English dropian "to drop" (see drip). Related: Drooped; drooping. As a noun, from 1640s.
A somewhat dull and stupid person: He's such a droop, he can't even discuss the weather intelligently (1930s+ Teenagers)