When macOS refuses to empty Trash with "The operation can't be completed because the item is in use," it means another process still has the file open, or the file itself is flagged as Locked.
Short answer: fully quit the app that opened the file (or restart the Mac to drop every file lock), then hold Option (⌥) while clicking Empty to override Locked-flag warnings.
Releasing the lock on a stuck file
macOS refuses to delete a file that is still memory-mapped by a running app. Closing a window is not enough — the app has to quit.
- open Trash and note which file is stuck
- identify the app that opened that file type (Preview for PDFs, QuickTime for video, Word for docs)
- right-click the app icon in the Dock and choose Quit
- if the culprit is unclear, restart the Mac — a reboot drops every file lock at once
- open Trash again and click Empty
A restart works almost every time because no background process survives it.
Forcing past the Locked flag
If the file is not in use but still refuses to delete, macOS has flagged it as Locked. Hold Option while emptying to override that flag.
- open Trash
- hold Option (⌥) on the keyboard
- click Empty in the top-right of the Trash window (or choose Finder > Empty Trash from the menu bar while still holding Option)
This tells Finder to ignore every Locked flag at once and delete the entire bin.
If a single file is the problem, right-click it in Trash, choose Get Info, and uncheck Locked before emptying normally.
Better next routes
If Trash empties but the drive is still full, large hidden folders are the likely culprit — continue with How to Find Which Folders Are Taking the Most Space (Mac).
For the broader system picture, use What is System Data on Mac?.