“Lay on, Macduff, and damned be him who first cries ‘Hold! Enough…”
The line is spoken when Macbeth and MacDuff are no longer friends. In fact, Macbeth is challenging MacDuff to a sword fight. The full phrase goes “Lay on, Macduff, and damned be him who first cries ‘Hold! Enough…” In other words, “Lay on, MacDuff” (not “lead on” as commonly thought) would equal “come at me, bro!”