"Before Lucy could begin to fraternize with the enemy, the police arrested her for being a spy."
Words in English such as "fraternize" come from the Latin word <em>fraternus, </em>meaning "brotherly." <em> Frater </em>is the Latin word for "brother." Think of how we refer to a group of "brothers" who live together in a "frat" house at a university as a "fraternity." To "fraternize" means to associate with someone in a friendly way. If you do that (or conspire to do that) with representatives of a country that is your own nation's enemy, that is considered to be spying or espionage.
Today the term in the news for "fraternizing" has been "collusion," as an investigation is underway as to whether members of the Trump presidential campaign team colluded with or fraternized with Russian operatives in order to sway the 2016 presidential election in the United States.
