1. an event/situation that makes it more likely something will fail
his/her latest mistake was another nail in the coffin of his/her career
This idiomatic expression is used to talk about the final action or event that ended or destroyed something that was already failing. Something that is ‘the final nail in the coffin’ makes it impossible or very unlikely to succeed. Similar in meaning to ‘the last straw’ or ‘the straw that broke the camel's back’. This is an informal phrase more likely to be used in social contexts.