The deba knife is a fish knife/cleaver for breaking down and filleting fish. The style is said to have originated around the end of the seventeenth century/beginning of the eighteenth century. Back in the Edo era this knife was just as likely to be used on a variety of birds and tortoises; the modern deba is primarily seen as a knife for fish and crustaceans that can also be useful for poultry.
Blade sizes range from the ko-deba at as little as nine centimetres or so through to great heavy monsters of thirty centimetres and longer. Professional-grade deba knives are kataha, which means single grind; left-handed and very large single bevel knives commonly require a custom order. Ryoba (double bevel) deba knives are easy to find and easier to sharpen.
The heft of the deba makes it easy to cut through fish bone that would prove stubborn or even damaging for a lighter blade. The deba blade is up to 1cm or more in thickness at the spine, and can perform as an effective cleaver without the swing - set the edge to the fish and give the spine a solid thump with the heel of your hand for a safe, controlled cut. Where the blade thins toward the tip it combines with the bevel to make a very fine point, ideal for precise filleting.
Why 'deba'? There's a story there.
The style first appeared in Sakai, historically and now one of the most productive knife-making centres in Japan. Sakai was a manufacturing centre for guns and swords during the sengoku jidai (the warring states period) - but times changed. With a unified Japan the market for weapons became less important and this led to many of the local kajiya (smiths) producing other kinds of edge tools. One of these Sakai smiths had a bad overbite. No, really.
In Japanese buck-teeth are 'de-pa' (出っ歯, out + tooth). Our kajiya with sticky-outy teeth created the first deba knife, and at the time it was referred to as a 'depa-bocho', after its creator. In what may have been a common sense play on words the knife eventually became a 'deba' (出刃, out + blade).
The story may be apocryphal - the creator was said to be 'a famous kajiya' but no-one seems to know his name - but it is widely accepted across Japan to be true.