Charles Ellicott Commentary Jeremiah 51:33

"For thus saith Jehovah of hosts, the God of Israel: The daughter of Babylon is like a threshing-floor at the time when it is trodden; yet a little while, and the time of harvest shall come for her." — Jeremiah 51:33 (ASV)

The daughter of Babylon ... —More literally, The daughter of Babylon is like a threshing-floor, in the time when it is trodden (i.e., when it is being prepared for the actual process), yet a little while, and the time of harvest shall come to her. The imagery is so familiar that it hardly needs an illustration (Isaiah 21:10; Isaiah 28:27–28; Micah 4:13).

The phrase “her harvest” in the Authorized Version is ambiguous. What is meant is that the heaped-up treasures of Babylon are merely like the harvest which shall be reaped by her conquerors, and the city itself as the threshing-floor on which men shall trample on the plunder.