Short version: it's a slightly obscene joke involving typographical humor.
Long version: In transliterated Modern Greek (sort of -- if I were being consistent, I would have used an I and become Ipogegrammeni), this is the name of the iota subscript, a diacritic created around 400 BCE by Greek scribes who were dealing with the fact that certain categories of diphthongs with long vowels were losing their I-ness. Rather than get rid of that superfluous I, they shrank it and began writing it beneath the 'main' letter: hence its English name, the iota subscript.
Thanks to the conservative nature of many many orthographies, and the particularly conservative bent of Greek orthography, this diacritic stuck around in the standard spelling of Greek until the 1960s (when its use was curtailed) and then finally 1982 (when a new, single-accent system for spelling Modern Greek was introduced). This, despite 1) the iota not having been pronounced for millennia by that point, and 2) the pronunciation of one of the vowels that could carry it having merged with no fewer than six other originally separate phonemes, so that there are six or seven ways in the modern language to write the sound pronounced /i/ [for more details see the Wikipedia entry on iotacism].
Now, purists being purists, and traditionalists being traditionalists, the iota subscript (and the circumflex, and the grave accent) didn't go down without a fight. Oh, no. And from this fight, we got the following humorous poem about the spelling reform, written in 1964 by Dinos Christianopoulos:
Μην καταργείτε την υπογεγραμμένη
ιδίως κάτω από το ωμέγα
Είναι κρίμα να εκλείψει
η πιο μικρή ασέλγεια
του αλφαβήτου μας
[Do not get rid of the iota subscript,
especially the one under the omega
It would be a crime to eliminate
this smallest obscenity
of our alphabet]
My nation's quote used to be an example of the iota subscript under the omega, but apparently NationStates doesn't like to play nicely with Unicode. O indeed! Here it is, in all its glory (plus bonus circumflex):
ῷ
