His ass.

Ok so just to nerd out for a second, plus ultra isn't just a snazy catchphrase, it's rooted in Latin; plus ultra translates to "further beyond" and is derived from the famous phrase "Nec plus ultra" meaning "nothing further beyond". Since antiquity the phrase "Nec plus ultra" was ascribed to the pillars of Hercules, which were thought to be the westernmost edge of the world and thus it was believed there was literally nothing beyond them.

So the catchphrase literally means "go beyond, further beyond" or more metaphorically "go beyond what you falsely believe is your limit".