The image depicts a human-like figure affixed to a cross and possessing the head of a donkey. In the top right of the image is what has been interpreted as either the Greek letter upsilon or a tau cross. To the left of the image is a young man, apparently intended to represent Alexamenos, a Roman soldier/guard, raising one hand in a gesture possibly suggesting worship. Beneath the cross is a caption written in crude Greek: Αλεξαμενος ϲεβετε θεον. ϲεβετε can be understood as a variant spelling (possibly a phonetic misspelling) of Standard Greek σέβεται, which means "worships". The full inscription would then be translated as "Alexamenos worships [his] God". Several other sources suggest "Alexamenos worshiping a god", or similar variants, as the intended translation.
No clear consensus has been reached on when the image was made. Dates ranging from the late 1st to the late 3rd century have been suggested, with the beginning of the 3rd century thought to be the most likely.
The graffito was discovered in 1857 when a building called the domus Gelotiana was unearthed on the Palatine Hill. The emperor Caligula had acquired the house for the imperial palace, which, after Caligula died, became used as a Paedagogium (boarding school) for the imperial page boys. Later, the street on which the house sat was walled off to give support to extensions to the buildings above, and it thus remained sealed for centuries.
The inscription is usually taken to be a mocking depiction of a Christian in the act of worship. At the time, pagans derided Christians for worshipping a man who had been crucified. The donkey's head and crucifixion would both have been considered insulting depictions by contemporary Roman society. Crucifixion continued to be used as an execution method for the worst criminals until its abolition by the emperor Constantine in the 4th century, and the impact of seeing a figure on a cross is comparable to the impact today of portraying a man with a hangman's noose around his neck or seated in an electric chair.
It seems to have been commonly believed at the time that Christians practiced onolatry (donkey-worship). That was based on the misconception that Jews worshipped a god in the form of a donkey, a prejudice of unclear origin. Tertullian, writing in the late 2nd or early 3rd century, reports that Christians, along with Jews, were accused of worshipping such a deity. He also mentions an apostate Jew who carried around Carthage a caricature of a Christian with ass's ears and hooves, labeled Deus Christianorum Onocoetes ("the God of the Christians begotten of an ass").
It has also been suggested that both the graffito and the roughly contemporary gems with Crucifixion images are related to heretical groups outside the Church.