The term "pcm" could refer to several things depending on the context. It might be an abbreviation for "parts per million," which is a measurement used to express very dilute concentrations of substances. It could also refer to "Pulsed Code Modulation," which is a method used in digital telecommunications. Without further context, it's difficult to specify its exact meaning.
The abbreviation "pcm" does not have a specific etymology as it stands for a longer phrase. "Parts per million" is derived from the combination of "parts," meaning portions of a whole, and "million," a unit of counting.
If "pcm" is a typo or an abbreviation for a specific term, please clarify for a more accurate response.