1) to throw a shadow upon, to envelop in a shadow, to overshadow

 From a vaporous cloud that casts a shadow the word is transferred to
 a shining cloud surrounding and enveloping persons with brightness.
 Used of the Holy Spirit exerting creative energy upon the womb of
 the virgin Mary and impregnating it (a use of the word which seems
 to have been drawn from the familiar OT idea of a cloud as
 symbolising the immediate presence and power of God)