Again, I will beg to differ on the assertion that K stands for Kompressor. It may have back in the days of the 540K like Greg mentioned. However, for the CLK or SLK, it means either kurz, klein or kompakt. They all work and make sense. On this german website
(underneath the rear shot with the Brabus plate), the last line of the paragraph it says "Er bedeutet sportlich, leicht, kompakt".
If it did mean Kompressor, the CL55 would be a CLK55, the SL55 would be an SLK55, the S55 would be a SK55, the E55 would be a EK55, the CLK320 and 500 would be a CL320 and 500, the SLK320 would be a SL320, and the CLK55 and SLK32 would be a CLKK55 and a SLKK32!