Why is this tagged 'cap'?

Because it is a cap?

"Cap" not as in "headpiece", but as in "still shot from a movie" ^^

"With thy finger, I make thoust explode."

"Thy" is second-person. What's fake Middle English for "my"?

comment #55146
Probably doesn't matter at this point.

comment #56721
Regardless, from what else I've heard I think it's "mine," as in "With mine finger..."

Then lobbeth thy Holy Hand Grenade at thine enemies who, being naughty in My sight, shall snuff it.