{Say} (eipon). Second aorist active imperative instead of
the old form eipe. The quotation is from  Isa 6:9,10 . This
very passage is quoted by Jesus ( Mt 13:14,15  Mr 4:12  Lu 8:10 )
in explanation of his use of parables and in  Joh 12:40  the very
point made by Paul here, "the disbelief of the Jews in Jesus"
(Page). See on Matthew for discussion of the language used. Here
the first time ("go to this people and say") does not occur in
Matthew. It is a solemn dirge of the doom of the Jews for their
rejection of the Messiah foreseen so long ago by Isaiah.