[Once he's done laughing, Rex continues to look in Shinichi's direction with an air of wicked glee.]

Ohhhh.

[Another breathy, raspy laugh follows.]

So you-

[Meaning Shinichi!

Rex makes it a point to single out the poor boy's embarrassed face at present.]

Definitely don't know what that mouth do.

[He circles the bespectacled schoolboy like a cat toying with a mouse, finally re-fixating his attention on Migi.]

Got it, got it, got it.

[From the get-go, every single one of Rex's motions is a dead giveaway that he is not the average human. For starters, he moves with a contradictory sort of agility and poise, like a cat or a panther, and is relatively light on his feet. It's an almost eerie-quality for someone so solidly built to have, but there is no question that each ridiculous gesture, from the wanton lift of a brow to the flick of a wrist, is meticulously controlled.]

So, for starters-

[He chooses to ignore the kid, instead more interested in the creepy weird thing, as its bold nature and ugly appearance strike him as oddly adorable-]

A dickhead is sort of the non-scientific term for the mushroom-shaped tip, of, well...

A dick. It's basically one of the many, many colorful ways to call someone an idiot. That's one of the great beauties of the English language! There's just so many different things you can say to somebody that's along the lines of, "Fuck you!"

But better!

[With an air of... Affection? Mean-spirited fondness? Rex's green-eyed gaze meets Migi's eye and remarks:]

You seem pretty sharp, though.