An assument is a patch or addition to something, a piece put on something for support or repair.
It seems like you may have meant "assumption," which is a belief or statement taken for granted without evidence. Assumptions can be useful for making predictions or decisions, but they can also lead to misunderstandings if they are not supported by facts. It's important to be aware of our assumptions and be open to questioning them.
It is an old word meaning a patch; an addition; a piece put on.