1. the offering of animal, plant, or human life or of some material possession to a deity, as in propitiation or homage.
2. the person, animal, or thing so offered.
3. the surrender or destruction of something prized or desirable for the sake of something considered as having a higher or more pressing claim.
4. the thing so surrendered or devoted.
Cain knew why God would not accept his Offering.
It was because it was not a true Sacrifice.
Abel had sacrificed his favorite sheep of the entire flock he kept, the sheep that he loved the most.
Cain would have to sacrifice what he loved most of all.
Or it would not be accepted by God.
"Where is Abel, thy brother?"
"I do not know. Am I my brother's keeper?"