As nouns the difference between lappet and lapel
is that
lappet is a small decorative fold or flap, especially of lace or muslin, in a garment or headdress while
lapel is each of the two triangular pieces of cloth on a suit which are folded back below the throat, leaving a triangular opening between.
As a verb lappet
is to decorate with, or as if with, lappets.