I never warmed to this phrase.  It means a youthful time, accompanied by carefree idealism, though recently in the U.S. it also means when someone is at the peak of their abilities.  People claim this makes sense because salads are fresh and wilt quickly.

Wikipedia says the phrase was coined by Shakespeare in 1606 in Antony and Cleopatra.  Cleo says “My salad days, / When I was green in judgment, cold in blood.”  There are many sites that agree: Shakespeare seems to make a reliable source.

The phrase became popular only from the middle of the 19th century and has had many literary uses.  Phrases.org notes that “Salad Days” was the title of a popular British musical in 1954.

It still leaves me cold.

NPR says the term filibuster “first appeared in the English language in 1591, according to the Oxford English Dictionary. The word was then `flee-booters’ [for ‘pirates’]. Its origins are probably Dutch, with some Spanish and French influences.” The pronunciation changed over time to our modern “filibuster”. In 1890 an obstructionist legislator was called a filibuster, and the word was on the road to our modern usage.

Online Etymology agrees with the origin of the word.

Free Republic offers more detail on the rise of our current usage.  In the early 1800s Senators began making long, irrelevant speeches on the Senate floor, which delayed or even stopped votes on legislation.  Battles over how to deal with the tactic ended in favor of the obstructionists, who were called filibusters by their opponents.  Today the old meaning is lost.  To filibuster is to disrupt and interfere with legislation through delay on the Senate floor.