* Literally: It could simply mean that the speaker is speaking in a literal way, not being figurative or metaphorical. For example, they might say "I'm not saying anything, my tongue is in my mouth" to mean they're literally not speaking.
* Nervousness: It could also be a playful way to say someone is nervous or hesitant to speak. They might be trying to avoid saying something that could get them into trouble, or they might be shy.
* Humorous: It might be used humorously to highlight a moment of silence or a lack of words. Someone might say "My tongue is in my mouth, I'm speechless!" to express surprise or amusement.
Ultimately, the meaning depends on the specific situation and how the phrase is used.
If you can provide more context, I might be able to give you a more precise interpretation!