As Arkay739 says, lacertus means "muscle," "arm," "male lizard." But I guess there's nothing wrong with that. Someone may well want a shirt that says "MUSCLE!" on it in Latin. Still, you probably should change the title.

the word Lacertus means "muscle" in latin, as in the muscle in your arm. It also means lizard. If you want it to mean "strength" in a metaohorical sense it has to be in the plural "lacerti" and even then the litteral word strength "vis" is better.