What is the origin of the idiom "down at heel" and when to use it?

The idiom "down at heel" originated from the literal condition of a shoe, where the heel is worn out or in poor condition. The phrase became popular in the 19th century and was often used to describe someone who had fallen on hard times and was unable to afford new clothes or shoes. Today, the phrase is still used in both British and American English to describe someone who looks scruffy or unkempt.