YOU SAID:

I have a codpiece in back

INTO JAPANESE

私は裏にコードピースがあります

BACK INTO ENGLISH

I have a cord piece on the back

INTO JAPANESE

私は背中に紐があります

BACK INTO ENGLISH

I have a string on my back

INTO JAPANESE

私は背中に文字列を持っています

BACK INTO ENGLISH

I have a string on my back