The word "intrepid" comes from Latin, combining the negative prefix in- ("not") with trepidus ("fearful, alarmed, trembling"), essentially meaning "not trembling" or "fearless".  It entered English via the French word intrépide.  It describes someone courageous, bold, and undaunted by danger.  It shares roots with trepidation, meaning fear or anxiety. 

According to the Oxford English Dictionary (OED), the adjective "intrepid" was first used in English in the late 1600s, with OED's earliest evidence coming from a 1697 translation by John Dryden.  Some synonyms are fearless, courageous, dauntless, or valiant, 

Examples of "intrepid" used in a sentence: 
"Super heroes are intrepid in their struggle for truth and justice." 
"Feeling particularly intrepid, we decided to have a climbing lesson." 
"Luckily, we have some brave and intrepid reviewers who are willing to take a stand." 
"Despite such warnings, one intrepid fan who did make the journey was quite impressed." 
"If you're feeling intrepid, the rugged interior of the park also offers challenging hikes and cycling." 
"We applaud the intrepid aviator that did this, although the actual details are yet to be confirmed." 

10 hours ago, drpaladin said:

Intrepid was the codename of Sir William Stevenson, Britain's head spymaster during World War II.

He was Canadian, from Winnipeg, I believe, but he worked for the British during WW2 and was a close friend and advisor to Churchill and Roosevelt. He lived a fascinating life, which is well worth reading. A self-made millionaire businessman, WW1 fighter ace, and the spy who many believe to be the inspiration behind Ian Fleming’s James Bond character. Fleming was trained in espionage techniques by Stephenson at Camp X, now called Intrepid Park, close to my first school, which was named after him.