What is a 8 letter word that antonym for weak with the 6 f and has ou or ow?

Here's why:

* 8 letters: "powerful" has 8 letters.

* Antonym for weak: "powerful" is the opposite of weak.

* 6th letter is "f": The 6th letter in "powerful" is "f".

* Contains "ou": "powerful" contains the letters "ou".