archaic or literary to lower (something, e.g. a flag), often as a sign of respect or submission
France must vail her lofty-plumed crest — Shakespeare
to remove one's hat as a sign of respect
to submit or yield
and Greek itself vail to our English voice — George Chapman
[Middle English valen short for obsolete avalen to let fall, from early French avaler, from Old French aval: see valance]