Is it ensure, insure, or assure?
Old English ofersceadwian "to cast a shadow over, obscure;" see over + shadow (v.). It was used to render Latin obumbrare in New Testament, as were Middle High German überschatewen, Middle Dutch overschaduwen, Gothic ufarskadwjan. Figurative sense is from 1580s. Related: Overshadowed; overshadowing.