down at heel vs down at the heels

down at heel

adj
  • Shabbily dressed, slovenly; impoverished; shabby, dilapidated. 

  • In poor condition, especially due to having worn heels; worn-out, shabby. 

down at the heels

How often have the words down at heel and down at the heels occurred in a corpus of books? (source: Google Ngram Viewer )