Bodies are believed to be material things, at least from the point of view of modern biology. The word body derives from an old English word, bodig, meaning trunk, or chest: a container. So bodies, generally speaking, are containers: for life, for intelligence. For consciousness.
(Swedenborg, incidentally, in Divine Love and Wisdom, refers to us as receivers of life... a very nearly perfectly stated analysis.)
Life, intelligence, and consciousness are ephemeral, intangible qualities that can be encountered in experience, but remain fundamentally inexplicable.
Bodies, in the world of Ibn al 'Arabi, are to be considered as vehicles for expression of the names, or attributes, of God. Intelligence certainly serves as one of those attributes, and furthermore, in his hierarchy, Knowledge (~intelligence) is superior to any other single attribute. Intelligence, in other words, is the supreme realization of the knowable, before one reaches the threshold of the Essence, the Reality, where knowing becomes impossible.
So when Gurdjieff speaks of the kessdjan body, the astral body, he speaks of a vehicle for intelligence. Man, in other words, is capable of forming a vehicle, a container, for a higher intelligence than the one he is accustomed to experience. The intelligence, we may presume, is electromagnetic in nature — as all intelligence and indeed all matter ultimately is. The difference here is that intelligence, an emergent property, can express itself in higher electromagnetic forms of organization that are independent of what we usually think of as bodies. And, indeed, in Beelzebub, we see the sun playing exactly this role with its emanations in regard to man.
On the level that Gurdjieff describes, the kessdjan body is the planetary body, that is, a body on the level of the planets. In the salient passages from In Search of The Miraculous, he goes on to describe two additional bodies, one on the level of the sun, and one on the level of all suns. Each body confers a different level of what man refers to as "immortality." Readers may refer to chapter 5, in which the subject is treated at considerable length.
In any event, the question remains as to why Gurdjieff referred to the astral body as the kessdjan body in Beelzebub.
The answer is actually not that complicated. Almost all of the unusual words in the book are in one degree or another derived from existing words and roots in Armenian, Russian, or Greek. This particular word, loosely translated, means "intermediate" body, or middle body. That is to say, a body that serves as a bridge between one state and another. And, indeed, Gurdjieff describes the astral body in more or less these terms, since what he refers to as real "immortality" — that is to say, existence after death for any lengthy period of time — is only conferred once one forms what he calls the mental body, which is on the level of the sun, and can survive the death of the astral body.
Gurdjieff's cosmology proposes nothing more than an extension of what we already know about bodies. Assimilating and organizing complex organic crystalline structures (molecules), which are themselves nothing more than electromagnetic relationships, bodies express emergent properties of God called intelligence, and (with more effort) consciousness. Consciousness, as I have pointed out many times, is measured according to level of complexity; all matter is to one extent or another conscious. Gurdjieff merely proposes that this process of concentration of cosmic substances continues on an invisible scale and extends itself into electromagnetic realms that we cannot directly perceive from our level, creating bodies that are capable of housing greater emergent levels of intelligence.
In this process, the astral body is only a stepping stone.
May your soul be filled with light.