мля

Что значит это выражение?

Damn, sh*t, f*ck, a common way to express frustration in colloquial speech. It is not to be used when talking to strangers or old people, because it is a bit vulgar. (As your american analogues above). It is used mostly in teens' lexicon, adult generation say it harder replacing the first letter with "б".

Thanks! is it derived from a verb or something?

Yeah, there's a word "бл..ь" which means "a whore, a slut", and also is an exclamation when the speaker feels frustration or anger whatever. As your "DAMN!!!", but it is very rude and intolerable in well-mannered society.

Also there is "блин", which is a softer version of "бл..ь" and can be used in ordinary common speach.
Not long ago teens started to use "млин" instead of "блин", although "блин" is used as well. Origins of "млин" are not familiar to me.

"бл..ь" often changes to "бля" throwing away the last two letters for easier pronunciation in common fast speaking.

"мля" is a soft replacement for "бля", when kids don't want to be blamed in using hardcore swearwords, but anyway everyone understands what "мля" is originated from.

Edited. L.

got it. thanks again