What is a prefix for punctual?

There isn't a commonly used prefix for "punctual." The word itself is already a complete concept, meaning "on time."

However, you could use prefixes to create related words:

* Unpunctual: This is the most direct opposite of "punctual."

* Prepunctual: This could be used to mean "before the expected time," though it is not a common word.

* Superpunctual: This could be used to emphasize extreme punctuality.

It's important to note that these prefixes are not standard and might be considered informal or creative in most contexts.