Is it ensure, insure, or assure?
c.1600, "to descend, sink," from lower (adj.), from Middle English lahghere (c.1200), comparative of low (adj.). Transitive meaning "to let down, to cause to descend" attested from 1650s. Related: Lowered; lowering. In the sense "to cause to descend" the simple verb low (Middle English lahghenn, c.1200) was in use into the 18c.
"to look dark and threatening," also lour, Middle English louren, luren "to frown" (early 13c.), "to lurk" (mid-15c.), from Old English *luran or from its cognates, Middle Low German luren, Middle Dutch loeren "lie in wait." Form perhaps assimilated to lower (1). Related: Lowered; lowering.
c.1200, lahre, comparative of lah (see low (adj.)).
Being an earlier division of the geological or archaeological period named. Compare upper.