Five by five is a radio communications expression that means 'loud and clear'. One of the fives represents the S units of reception strength. The other five is a rating of the signal clarity. Five by five is a good, clear signal. The radio use of this expression goes back to the 1950s: "'All right, testing, one-two-three-four. . .' 'Five by five, Mr. Holloran!'" (Hunter,
Blackboard Jungle, 1954).
The more metaphorical meaning, popularized by
Faith Lehane from the television show Buffy the Vampire Slayer, wasn't recorded until the 1980s.
The use of the word is primarily used in the military, or when discussing something similar.
