A tunic is a garment that was worn in Antiquity (both in ancient Greece and Rome — the word tunic comes from Latin tunica). It was worn by men and boys, citizens, slaves and soldiers, and some forms of tunic were also worn by females.
The tunic is a kind of loose fitting gown that was usually tied around the waist with a belt. Tunics came in varying lengths, from ankle-length (rare) to just-above-knee-length (most typical) up to tunics that barely covered the hips. They were cut wide enough at the legs to permit running and other physical exercises — for things such as horseback riding, shorter tunics were worn. The length of the tunic, the presence or lack of stripes, as well as their width and ornamentation, would indicate the wearer's social status. Tunics could be sleeveless, short sleeved or (rarely) long sleeved.
In Antiquity, the tunic was worn either as an undergarment (e.g. under a toga or stola), or several tunics were worn on top of another. There was no underwear worn under a tunic, so for toilet use and similar things the hem of the tunic could simply be lifted like a skirt.
Mention in spanking literature
In the spanking novel The Old Rectory, the boys of the institution are given lessons in Greek dance, wearing nothing but short tunics that barely cover their frontal private parts but more or less completely expose their bare buttocks, especially when they have to bend forward in dance poses.