Simply put, greek sex is a synonym for anal sex. The term itself is considered to be a euphemism for anal intercourse, therefore is also known as greek anal.
The name greek sex derives its origins from the ancient Greeks, who were in fact very accepting of sexual and romantic relationships between men. Outside of Greece, however, the anal sex as a sexual act had not been really accepted by other cultures worldwide. Because the Ancient Greeks were known for endorsing homosexual sex, they were a continuous target of mockery in Roman plays and comedies. In these activities, the Romans often employed Greek characters and settings for explicit scenes where anal sex was involved. By doing this, the greek anal gained its reputation of being a practice that only the Greek culture recognizes.
The interesting part of the name's origin is that the Ancient Greeks did not have words for the terms heterosexual and homosexual. Back then, there was not a single concept that was equal to the notion we know of today. It was assumed that every greek experience the feelings of heterosexuality and homosexuality in the course of their lifetime, only in different periods. The sexual passion and romantic love between male partners were considered not only normal but also very healthy. Thus, the greek anal was commonly practiced among the Greeks, without any judgment.
Today, greek sex is typical for homosexual relationships between two or more men. Since the anal opening is a very delicate and sensitive area, if performed correctly, can be a highly enjoyable sexual practice. During greek anal, the participants are recommended to start slow and always use lubricants to enhance their experiences.