Not sure where that snuck in from, some grey matter somewhere has it lodged.
Perhaps you've been around a while?
From Wikipedia:
Maritime/Amateur VHF prowordsRadio Check
What is my signal strength and readability; how do you hear me?
5 by 5 is an older term used to assess radio signals, as in 5 out of 5 units for signal strength and for readability. Other terms similar to 5x5 are "loud and clear" or "Lima and Charlie". Example:
ALPHA 12: X-RAY Two-Three, THIS IS ALPHA One-Two, RADIO CHECK, OVER X-RAY 23: ALPHA One-Two, THIS IS X-RAY Two-Three, I READ YOU 5 BY 5, OVER ALPHA 12: ALPHA One-Two ROGER, OUT US Army correct check:
ALPHA 12: X-RAY Two-Three, THIS IS ALPHA One-Two, RADIO CHECK, OVER X-RAY 23: ALPHA One-Two, THIS IS X-RAY Two-Three, ROGER OUT If the initiating station (ALPHA 12 in the example) cannot hear the responding station (X-RAY 23 above), then the initiator will attempt a radio-check again, or if the responder's signal was not heard, the intiator will reply to the responder with "NEGATIVE CONTACT, ALPHA 12 OUT".
The following readability scale is used: 1 = Bad (unreadable); 2 = Poor (readable now and then); 3 = Fair (readable but with difficulty); 4 = Good (readable); 5 = Excellent (perfectly readable).
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