Slang
Slang is what you learn when playing an American online game.
Or listen to music.
Examples:
- Love me tender/ sweet/ long/ true (Elvis Presley: Love me tender) ("long" is a flat adverb, the others are not)
- Words don't come easy (song by F.R. David) (though already listed as "flat adverb", this one also really feels like slang)
- "don't make it bad" (The Beatles: Hey Jude) (not even entirely clear to native speakers what is meant by that)
- "Ima do this real quick" (meaning "I am going to do this really quick(ly)" - quick is an optional flat adverb, the rest is slang)
- you hurt me bad (American informal - should be "badly" or "very much")
- you're plain stupid (should be "plainly")
- he's a full grown idiot (should be "fully grown", IMHO)
- I want you so bad (American informal - should be "badly" or "so much")
- Don't talk soft (for once "British informal" - featuring a possible flat adverb and meaning "Don't say such silly things!")
- this is bloody perfect (offensive language doesn't need a bloody suffix)
"Running wild"
There is a song "Running wild" by Marilyn Monroe:
That's the reason I'm
Runnin' wild, lost control
Runnin' wild, mighty bold
There is also a song "Running wild" by Judas Priest:
I move amongst the nightlife
And they just step aside
'Cause when they see me coming
They know I'm running wild
...
Get outta my way... I'm running wild
Based on those lyrics I think we can assume that it means "I am freaking out/ losing control" and can be compared to "going crazy",
so that "wild" would here indeed be an adjective. It is still a curious phrasing (to me at least).