Jamieson, Fausset and Brown on 2 Peter 1:14

14. shortly I must put off--Greek, "the putting off (as a garment) of my tabernacle is speedy": implying a soon approaching, and also a sudden death (as a violent death is). Christ's words, Joh 21:18, 19, "When thou art old," &c., were the ground of his "knowing," now that he was old, that his foretold martyrdom was near. Compare as to Paul, 2Ti 4:6. Though a violent death, he calls it a "departure" (Greek for "decease," 2Pe 1:15), compare Ac 7:60.