Learn Russian: Это совсем не смешно!

When совсем is placed directly before не, the phrase совсем не acts as an intensifier meaning "not at all." If you think of it literally as "completely not," the logic makes sense, but "not at all" is how we express this in natural English.

What would it mean if I swapped the word order to не совсем смешно?

The meaning would change completely! While совсем не means "not at all," не совсем means "not exactly" or "not entirely." So, Это не совсем смешно would mean "That's not exactly funny" (implying it might be meant as a joke, but it missed the mark), rather than "That is not funny at all."

Could I use вообще не instead of совсем не in this sentence?

Yes, absolutely. Это вообще не смешно! means exactly the same thing and is extremely common in everyday conversational Russian. Using вообще often adds a slightly stronger, more emotional emphasis to your statement.

Why do we use the neuter short form смешно instead of the full adjective смешной?

When you use это as a standalone pronoun (meaning "this is" or "that is") to describe a general situation, the describing word that follows must be the neuter short form or adverb form ending in -о. You would only use the full adjective смешной if it were directly modifying a masculine noun, like этот смешной парень (this funny guy).