c. 1600, "descend, sink, grow less or lower" (intransitive), from lower (adj.), comparative of low (adj.). Transitive meaning "let down, cause to descend" attested from 1650s. Related: Lowered; lowering. In the transitive sense "to cause to descend" the older verb was low (Middle English lahghenn, c. 1200), which continued in use into the 18c.
"to look dark and menacing," also lour, from Middle English louren, luren "to frown, scowl" (early 13c.), "to lurk" (mid-15c.), from Old English *luran or from its cognates, Middle Low German luren, Middle Dutch loeren "lie in wait." The form perhaps has been assimilated to lower (v.1). Related: Lowered; lowering.
Middle English lawar, lower, lougher, earlier lahre (c. 1200), comparative of lah "low" (see low (adj.)). As an adverb from 1540s. Lower-class is from 1772. Lower 48, "the forty-eight contiguous states of the United States of America, excluding Alaska and Hawaii," is by 1961 in an Alaska context (Hawaii actually is "lower" on the globe than all of them).