Is it ensure, insure, or assure?
Old English scimerian "to glitter, shimmer, glisten, shine," related to (perhaps a frequentative of) scimian "to shine," from Proto-Germanic *skim- (cf. Swedish skimra, Dutch schemeren "to glitter," German schimmern), from PIE root *skai- "to gleam, to shine" (see shine (v.). Related: Shimmered; shimmering.
"a tremulous light," 1821, from shimmer (v.).