Jack is a name that came to mean everybody or anything common. As in:

Lumber-jack

Flap-jack

A mad jack and a swearing rubsby. (From Shakespeare for a nutty guy)

Jack-Knife.

Every man jack of them.

Just an odd cultural thing

Jake – A term used to refer to a good firefighter. One story has the term originating from the corruption of the phrase “J-Key”. Years ago, the bed was the most expensive piece of furniture in a dwelling. Since firefighters were not as good at stopping fire as they are today, tehy would attempt to remove as many pbelongingas as possible. the bed was too large to carry out in once piece and had to be disassembled using a bed key. they key was shaped like the letter J and was called a J-key