Soap residue, also known as soap SCUM, is a film or layer of solidified soap that is left behind after washing. It is usually composed of minerals, fats, and oils that have been combined with the soap. The two crossword solutions for this term are SCUM (4 letters) and NIGRE (5 letters). SCUM is a derogatory term for someone who is considered to be of low moral character, while NIGRE is an archaic term for something that is black or very dark in color. Both terms can be used to describe soap residue, which is often dark in color and can be seen as unsightly.