What does "sweat blood" mean?

Also, sweat one's guts out. Work diligently or strenuously, as in The men were sweating blood to finish the roof before the storm hit. The phrase using guts was first used about 1890, and that with blood shortly thereafter.

Suffer mental anguish, worry intensely, as in Waiting for the test results, I was sweating blood. This usage was first recorded in a work by D.H. Lawrence in 1924. Both usages are colloquial, and allude to the agony of Jesus in Gethsemane (Luke 22:44): “And being in an agony he prayed more earnestly: and his sweat was as it were great drops of blood falling down to the ground.”