How do you say Antemeridian?

"Antemeridian" is already the word. It's a fancy way of saying "before midday" or "before noon." It's often abbreviated as "A.M."

So, if you want to say it, you just say "Antemeridian."

For example:

* "The meeting is scheduled for 9:00 antemeridian."

* "I wake up before the antemeridian."

However, in everyday conversation, people usually just say "A.M." or "in the morning."