Your gold and silver is cankered; and the rust of them shall be a witness against you, and shall eat your flesh as it were fire. Ye have heaped treasure together for the last days.
Is cankered - rusted through.
Rust ... witness against you - in the day of judgment; namely, that your riches were of no profit, lying unemployed, so contracting rust.
Shall eat your flesh. The rust which once eat your riches shall then gnaw your conscience, accompanied with punishment which shall prey upon your bodies forever.
As ... fire - not with the slow process of rusting, but with the swiftness of fire.
For the last days - ye have heaped together, not treasures, as ye suppose (cf. Luke 12:19), but wrath against the last days. Rather, 'In the last days (before the coming judgment) ye laid up treasure' to no profit, instead of seeking salvation (see note, James 5:5).