Definition from Wiktionary, the free dictionary
Etymology 1 
- (archaic) The kidneys.
- The lower part of the back.
- (architecture) The parts of a vault between the crown and the spring or abutment, including, and having especial reference to, the loading or filling behind the shell of the vault.
Etymology 2 
- Plural form of rein
- Third-person singular simple present indicative form of rein.
reins m pl (singular rein)