Face melting is what happens to the crowd at a hard rock/heavy metal concert. Its a by product of "the rock" and "rocking out" usually brought about by a nasty guitar solo or killer breakdown. Can include other facets of heavy metal music, like "high voltage", "explosives", "damnation" or the like. Also heard in the iteration, "you melted my face." Bands known for face melting are AC/DC, Iron Maiden and Judas Priest, however Rush has been known to melt faces, and Eddie Van Halen of Van Halen is reported to have an effect pedal that generates "face melting" sound. The only real case of (out of context) face melting on record is from "Indiana Jones and the Raiders of the Lost Ark" where at the end, Nazis get melted. Also, the "World of Warcraft" community has borrowed the term for a particular sequence in their video game.
Malcolm Young says to Angus Young, "Lets get out there and commence with the face melting!"
The condition in which, due to an extreme exposure to an event of epic Awesomeness, Horror or any other emotion on the more extreme end of the spectrum of emotions, one loses all perception of space and time including (but not limited to) a brief lapse in physical awareness. Such an emotional rush can even override Pain, which in some cases may be the cause of the rush.
In addition to being common among the disciples of Metal and among those who get so stoned that they can't feel certain parts of their bodies (such as their faces), it takes on somewhat of a more literal sense if one happens to plaster a White Phosphorus or Acid Grenade on the face of someone annoying them (some Nazis in a certain famous movie learned this the supernaturally hard way...in any case, you really shouldn't mess with White Phosphorus or dangerous Acids except for in Video Games).
While they writhe in pain, it may be customary to say the hilarious-but-somewhat-stupid phrase below:
Enemy n00b: "ARRGH my head asplode!"
You: "Would you like a Guitar Solo with that Face Melting!?" (proceeds to mime awesome solo on Air Guitar)