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"