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.

A swimmer

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.

To be in a content state of being. To be fine

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.

a short, fat dude

A small one room economy flat.