Back then its meaning was clear to all: Even the most casual, innocent sounding discussion might give valuable information to enemy spies.
It seems that Hillary Clinton and her associates never heard this expression or didn't feel it worth consideration. Perhaps they should have.
Within the gaggle of emails hosted on H. Clinton's servers, unclassified servers, was contained a back and forth between persons discussing a certain nuclear scientist from Iran who had, supposedly, been helping the United States.
Of course WikiLeaks had captured these emails and made them public. Now, that scientist is dead, executed by Iran for treason. How did iran find out? Through H. Clinton's emails, that's how. Click Here for the Story.