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1.
This is an old aeronautical radio term. Radio signal strength and clarity were measured on a scale of 1 to 5. Today the term "Loud and Clear" is preferred to "5 by 5".

In slang use "5 by 5" means things are well, good, or as expected.
by balrog May 16, 2003
 
2.
Respective rating used to report ballpark signal strength and clarity of a transmission. So a 5 by 1 means a strong signal is present, but what is saying cannot be understood. 1 by 5 means you can understand them, but a signal is barely coming in. 5 by 5 is ideal for both counts.

Can also be used nowadays to mean one is doing well.
Ex. A) – command to alpha2IC, do you read?
-command this is alpha2IC, I got you 5 by 5.

b) How’s it going?
I’m 5 by 5.
by vodka June 02, 2004
 
3.
"In the Pipe, 5 by 5"

This is a specific reference to one of several conditions.

1: You have entered a large concrete pipe and everything is ok
2: You are piloting a starship into the atmosphere of a planet and are in the perfect trajectory pipe. Everything is ok.
3: You have entered a vagina and everything is ok.
4: You have entered a vagina and its dimensions are approximately 5 by 5"
Example 1
You are being chased by a tyrannosaurus rex and climb into a large concrete pipe such as those found on the island of Isla Nublar where Jurassic Park is. Upon entering you hear Doctor Hammond's voice on the radio.
"John, John, are you alright?"
"5 by 5 Hammond, I'm in the pipe and everything is five by five"

Example 2
You are piloting a human dropship carrying a platoon of marines, such as those seen in Aliens and Starship Troopers. As you hit the atmosphere, the dropship encounters a normal amount of turbulence. On the radio the mothership asks for a report.
"Dropship 7-1-Alpha, what is your status?"
"We're in the pipe, 5 by 5"

Example 3
You have been dared to mount a brontosaurus type lady by your charming friends. Upon seducing her with your willy ways and copious amounts of alcohol you take her to a room. Upon stripping her, laying her on the bed and opening her legs, you enter her cavernous vagina. Just then you're phone rings.
"What's going on dude!!!"
"Hahahaha, I'm in the pipe, it's 5 by 5!!!"
by R.J. O'Brien July 03, 2013
 
4.
To be in a content state of being. To be fine
Hey how are you? 5 by 5.
by Xander October 29, 2003
 
5.
Also refers to level of orientation in a neurological/psychiatric exam. Five by five means that you are oriented to (your)name, place (where you are), time, day/date, and year/current President. A way of saying that you're alright, everything's cool.
Dood- are you allright, that was one ugly wreck.

It's cool, I'm five by five.
by anoynymous April 23, 2004
 
6.
A small one room economy flat.
A tiny home, with small dementions that has everything including a cooking area, living space, and sleeping quarters in the same room. Usually the bathroom has it's own separate room or area that is closed off from the rest of the humble abode.
by Tristan December 21, 2003
 
7.
a short, fat dude
the fat ass was 5 by 5!
by 0000 December 04, 2003