Other forms: reins, reining, reined
A rein is the strap by which a rider guides a horse. One in each hand, running to the bit in the mouth.
It is nearly always plural in use. The figurative senses are numerous and all about control: taking the reins means taking charge, keeping a tight rein means controlling closely, and giving free rein means letting somebody do as they like. Rein in means to restrain. Take care with the spelling, since reign means to rule and rain falls from the sky, and all three sound alike.
A rein holds back. Latin retineō joins re, back, to teneō, to hold, which means that reining something in is the word saying itself twice over.
Latin
retineō — hold back
la-eme
retina
Anglo-Norman
resne
Middle English
rein
English, today
rein
Built from
From Middle English rein, reyne, from Anglo-Norman resne, from la-eme retina, ultimately from Classical Latin retineō (“hold back”), from re- + teneō (“keep, hold”). Compare modern French rêne. Displaced native brīdel, wealdleþer, ġewealdleþer (“rein, bridle”) literally “control strap”, sāl, tiġel, and lāttēh, lāttēh (“leash, rein”).
Etymology from Wiktionary, CC BY-SA