'unfused' is formed in modern English by adding the negative prefix 'un-' to 'fused'. The verb 'fuse' ultimately comes from Latin, specifically the verb 'fundere' meaning 'to pour'.

The Latin verb 'fundere' ('to pour') passed into Old French as 'fuser' and Middle English as 'fusen'/'fuse', developing the sense 'to melt or join by heat'; the English negative prefix 'un-' was later attached to form 'unfused'.

Originally related to pouring or casting liquids ('to pour'), the root developed the specialized meaning 'to join or melt together' in English; 'unfused' therefore came to mean 'not joined by melting or welding'.