If you sent something inappropriate to someone on facebook messenger and now you want to unsent it then, don’t worry, just follow these steps.
- Go to your messenger application
- Now, visit the thread where you want to unsend the message
- Tap on the message and hold your finger for a few seconds
- Then, an Unsend option will appear in the middle of the screen
- Tap on it to unsend the message
Source: https://getassist.net/how-to-see-unsent-messages-on-messenger/