System D (in French, Système D) is a shorthand term that refers to a manner of responding to challenges that requires one to have the ability to think fast, to adapt, and to improvise when getting a job done.
The letter D refers back to either of the French nouns "débrouille", débrouillardise or démerde (French slang). The verbs se débrouiller and se démerder mean to make do, to manage, especially in an adverse situation.
In Down and Out in Paris and London, George Orwell calls out the term "débrouillard" as something the lowest-level kitchen workers, the plongeurs, wanted to be called, as people who would get the job done, no matter what.
The term gained wider popularity in the United States, after appearing in the 2006 publication of Anthony Bourdain's The Nasty Bits. Bourdain references finding the term in Nicolas Freeling's memoir, The Kitchen, about Freeling's years as a Grand Hotel cook in France.
In recent literature on the informal economy, System D has become a shorthand name for the growing share of the world's economy which makes up the underground economy, which as of 2011 has a projected GDP of $10 trillion.
There is a range of terms in other languages describing similar circumstances, examples for those are Trick 17 in German, Trick 77 in Swiss German, Trick 3 (kikka kolmonen) in Finnish and to hack it in English, Jugaad in Urdu, Hindi, and Punjabi.