423 Locked
Meaning
The resource that is being accessed is locked and cannot be modified.
When to Use
When a WebDAV operation is attempted on a resource that has been locked (by a lock mechanism in WebDAV) and the server is refusing to proceed until the resource is unlocked.
Example
A WebDAV client trying to DELETE a file that has an active lock (by another user/process) might receive 423 Locked, indicating the file is currently locked.
When Not to Use
Do not use if the concept of locking is not in play. Outside WebDAV or similar scenarios, this code is not commonly used. Use other appropriate 4xx codes if the resource isn't accessible for different reasons.
Source
RFC 4918 (WebDAV)