Generally used to designate a poorly drawn smiley in MSN messenger that may or may not look like a fly. In reference to a Jack Handey Deep Thought that ended with "That alien!", but written in english shitty enough to make you invert singular and plural and put the letter D instead of the "th" sound.
MikeK: Mans! Loks at my nwew smilyeS!
MikeK: <shitty smiley>
D: whtat the fkucs is that supposeded to be?! it looks like a fruit flies or smomethinigs!11
MikeK: Dat Flies