1) To hinder, obstruct, prevent or stand in the way of.
1419 rutes, wedys and erthe that hafes ben casten thar out of thaire gardyns or thayre houses ... the whilk lettys the water to hafe the ryght issue, York
1482 the said viage had beyn lettid for that seson, York
1569 Richard Hyrste ... was throughe a greate snowe letted and stopped, Almondbury. It was used also as a noun and adjective: 1587 John Kaye ... shall have one convenyente waye ... for dryvinge and fetching of cattell ... without let of John Baylie, Thurstonland
1642 findinge of imployment for our moore folks when there is any lette weather in harvest time, Elmswell.