socks and knocks n.pl. the oxides of sulfur and nitrogen commonly found in pollution. Etymological Note: A literal pronunciation of the characters that make up the chemical notations SOx, representing various oxides of sulfur, and NOx, representing various oxides of nitrogen. A third related term, ROx, is explained in the 2001 citation. All of the lower-case exes in the chemical notations, properly represented, would be subscript. (source: Double-Tongued Dictionary)
Further reading
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