Have my back means "defend me"
Back into the fight. Could also mean back into the struggle of life.
go back in time
It means "way back when"
you will never go back to it
Generally speaking, the phrase "back to back" means two events which occurred next to each other, which are intended to be visited or achieved separately and independently of each other.
To put on the back burner means to delay doing something till later.
back
Sit back and enjoy it.
Vuelve a mi translates from Spanish to English to mean back to me. It is typically seen in the phrase 'Vuelve mi amor por favor' which translates into come back to me my love.
The phrase "crossing the Rubicon" has come to mean an irrevocable move, something you cannot take back.
A sheep fold is a sheep pen, a place of safety. The phrase is to welcome someone back after they have been away.
The abbreviation "AP" in the phrase "AP diameter" means from front to back.