An expression which comes from writing the sentence "Calm the fuck down" as if it were French: "Comme" for Calm, "de" for the, and Fuckdown altogether. It is probably trying to make fun of both
English slang (da/de used for "the") and French language (or, of French people that don't know how to write
English properly).
The expression "Comme de Fuckdown" was made
popular by a Dutch apparel company producing caps, t-shirt and goodies displaying that sentence.