Spam is an example that any email user knows about – this word was originally a metaphor, from 'Spam', a type of canned meat. Servers putting unwanted email into somebody's inbox was similar to waiters putting unwanted Spam into food. This was originally suggested by a Monty Python scene. When we use a metaphor very often and we forget the old meaning, or forget that the two meanings are connected, this is a 'dead metaphor'.