1. The highest rank of bro. To refer to someone as your Leftenant Bredren is to refer to them as your best friend or wingman.
2. Right hand man.
CAN ALSO BE SHORTENED JUST TO LEFTENANT in casual form
"Hey leftenant how you doin today?"
bro rankings range from Maggot Bredren (someone you hate) to Leftenant Bredren (you're boy)
"Thanks for the help with that girl, you're a true Leftenant Bredren"
"You helped me out big man, you're officially promoted to Leftenant Bredren"