A.T. Robertson Commentary Matthew 20:23

"He saith unto them, My cup indeed ye shall drink: but to sit on my right hand, and on [my] left hand, is not mine to give; but [it is for them] for whom it hath been prepared of my Father." — Matthew 20:23 (ASV)

Ye shall drink (πιεσθε). Future middle from πινω. Christ's cup was martyrdom. James was the first of the Twelve to meet the martyr's death (Acts 12:2) and John the last if reports are true about him. How little they knew what they were saying.