Pretty Sure It's "Thepump"

Woke up this morning trying to remember Optimus Prime’s last name

Uhm....I believe it's "Prime." 

Well, if you go with the assumption that Cybertronian transforming robots follow the same naming conventions that many Terran families do, it's a fairly safe bet that the Autobots and/or Decepticons do actually have surnames.  Otherwise, it's a bit of a stretch to think that buried somewhere in the Transformers lore, the characters have last names that have simply been ignored by all of the canon.  

This is Afternoon Jeremy's explanation for whatever was going through my head earlier today.  

It's been well established that I Am A Genius, but it has probably also been brought up that I don't function properly in the morning.  My genius is like a diesel engine, in that it's pretty lousy until I get warmed up, and then I'm ready to roll for a good long time.  So, first thing in the morning (and I mean FIRST thing...as in, I woke up with this question in my head, and hadn't even sat up in bed yet), I was totally preoccupied with the fact that I couldn't remember what Optimus Prime's last name was.  And I don't mean "Prime," either...that part I was totally clear on.  I thought there was something else going on afterwards that wasn't very often used.  For some reason, I was bouncing around with the word "Freeze" or something that makes no sense.  It was pretty moronic.  

Granted, there are various portions of the Transformers canon that refer to Orion Pax as an alternative name for Optimus Prime, but I'm not overly concerned about that.