The deba knife is a knife/cleaver for breaking down and filleting fish. Its heft and robust edge make it possible to safely cut through fish and poultry bones that would prove stubborn or damaging for a lighter blade.
The blade of a deba can be 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 wide 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.
Blade sizes range from the ko-deba at as little as nine centimetres or so through to great heavy monsters of thirty centimetres or longer. Professional-grade deba knives are kataha, which means single grind; left-handed and very large knives commonly require a custom order. Ryōba (double bevel) deba knives are easy to find and easier to sharpen.
Back in the early Edo era this knife was just as likely to be used on birds and tortoises; the modern deba is seen as a knife for fish and crustaceans that can be used to break down poultry or other meats. The deba is not used to break through heavy meat bones.
Why 'deba'? Here's a little background.
The style originated in the eighteenth century 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 collapsed and this led to many of the local kajiya (smiths) producing other kinds of edge tools. To give an idea of the scale of things, in 1750 Sakai there were 64 kajiya producing kitchen knives and 37 kajiya producing blades for tobacco. One of them 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, at the time 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 - everyone says the creator was 'a famous kajiya' but no-one seems to know his name - but it is widely accepted across Japan to be true. So there you go.