Here's why:
* "Slip of the tongue" is a common phrase meaning an unintentional mistake in speaking.
* "Slip of the foot" isn't a common phrase, but it could be interpreted as making a mistake with your feet, like tripping or stepping wrong.
It's possible the phrase you are looking for is "better slip with the tongue than with the foot" or "better to slip with your tongue than with your foot". This saying means it's better to accidentally say something wrong than to accidentally hurt yourself physically.
It's important to note that this saying can be interpreted in different ways, depending on the context. It could be used to encourage caution in speech, to highlight the importance of physical safety, or to emphasize the difference between verbal and physical mistakes.
If you can provide more context about where you heard this phrase, it may be possible to understand its intended meaning more accurately.