Now, I am certainly no [phraseologist], ..., well, maybe I am somewhat of an [armchair quarterback|armchair phraseologist], in that occasionally I will speak or hear some [colloquialism] or another and think to myself something along the lines of, "[I wonder] who first said that, why, and exactly what it originally meant?" And usually it turns out to be from either [William Shakespeare |Shakespeare] or [The Bible], but I still usually use [Google] to help me track these things down.

In the case of "[scum] of [the earth]", as far as I can tell, it's a Bible one. Well, [sort of], anyway... The phrase "[slime of the earth]" appears in [Genesis 2], verse 7, but only in the [Douay-Rheims Bible|Douay-Rheims translation of the Bible], which, as many people who profess to know about that sort of thing would point out, is neither terribly accurate nor modern. In pretty much every other translation, in place of "slime of the earth" appears "[dust of the ground]", as in, "And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul."

So, to sum up, the phrase probably entered into the [English] language in [1609] (which was when the Douay-Rheims translation was first published), at some later point "[scum]" replaced "slime", because [let's face it], "scum of the earth" just [roll off the tongue|rolls off the tongue] more nicely than "slime of the earth", but since it really means "dust of the earth" anyway, can probably be expressed as the roughly-equivalent "[dirtbag]".

or something.