Scum \Scum\, v. i. To form a scum; to become covered with scum. Also used figuratively.

Life, and the interest of life, have stagnated and scummed over. --A. K. H. Boyd.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Scum \Scum\, n. [Of Scand. origin; cf. Dan. & Sw. skum, Icel. sk?m, LG. schum, D. schuim, OHG. sc?m, G. schaum; probably from a root meaning, to cover. [root]158. Cf. Hide skin, Meerschaum, Skim, v., Sky.]

1. The extraneous matter or impurities which rise to the surface of liquids in boiling or fermentation, or which form on the surface by other means; also, the scoria of metals in a molten state; dross.

Some to remove the scum it did rise. --Spenser.

2. refuse; recrement; anything vile or worthless.

The great and innocent are insulted by the scum and refuse of the people. --Addison.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Scum \Scum\, v. t. [imp. & p. p. Scummed; p. pr. & vb. n. Scumming.]

1. To take the scum from; to clear off the impure matter from the surface of; to skim.

You that scum the molten lead. --Dryden & Lee.

2. To sweep or range over the surface of. [Obs.]

Wandering up and down without certain seat, they lived by scumming those seas and shores as pirates. --Milton.

Source: Webster's Revised Unabridged Dictionary (1913)