1) This phrase refers to any event where innocent
person(s) were
taken quietly and unassumingly to their deaths without the people who know them discovering about it until after it happened.
2) It is also a short story by Roald Dahl, the
author of charlie and the chocolate factory.
"The
Jews were lead to the showers like lambs to the
slaughter during WWII"
"At Gethsemane, Christ knew he had to be
led like a lamb to the sluaghter"