What does disaster mean in the following sentence?
I'm a disaster.
Check this dictionary entry at Oxford:
noun: a sudden accident or a natural catastrophe that causes great damage or loss of life: 159 people died in the disaster
[mass noun]: informal a person or thing that is a complete failure: lunch had turned out to be a total disaster
So, it is generally (may change on context) used to say how messed up (or hopeless) as a person the speaker is.
Generally, a person will say this if he/she
In my opinion, the most important thing for understanding the words, their exact meaning, is to know their root - their etymology. In this particular case, the word disaster has its origin from Latin words dis which means bad and aster which means a star. Therefore disaster is a bad star! Our predecessors relied on astrology and they believed that the positions of stars can tell something about our life, about our destiny…