formless describes what has no regular shape or defined structure — lacking order and arrangement, or lacking physical existence altogether.
The word does different work depending on which sense is active. Applied to matter it is merely descriptive: fog, smoke, a heap of sand. Applied to something made — an essay, a piece of music, an organization — it is a criticism, naming the absence of a structure the thing was supposed to have. Applied to what is immaterial it is neither praise nor complaint, simply noting that the subject has no body in which to take a shape.
A fourth reading treats formlessness as a capacity rather than a deficiency, and it is the reading that makes the word worth keeping. To be formless in this sense is to be unfixed, and therefore able to take the shape of whatever situation arrives.
"Empty your mind. Be formless, shapeless; like water." ~ 📝Bruce Lee
That line comes from the passage collected as 📝Be Like Water, where formlessness is a discipline rather than a failure of form. Water is not shapeless because it lacks structure — it is shapeless because it yields to the vessel, and, given enough time, wears the vessel away.