I came across this phrase while looking up the term Prague Summer, a phrase used in Jeffery Toobin’s The Nine. The Prague Spring, according to Wikipedia, was “a period of political liberalization in Czechoslovakia during the era of its domination by the Soviet Union after World War I.” In January 1968, with the rise of power of Alexander Dubček, Czechoslovakia began a period of reform to decentralize the economy and grant democratic rights. By August, the Soviet Union and other Warsaw Pact members invaded the country and put an end to the reforms.
Toobin used his version of the phrase to refer to the politcal liberization of Supreme Court Justice Anthony Kennedy. Kennedy,after spending a summer abroad working with international courts, returned the SCOTUS with a much greater leftward shift.