The term 'Tired Old Horse' can be summarized by two crossword solutions: NAG (3 letters) and JADE (4 letters). NAG is a verb meaning to annoy or irritate someone with persistent complaints or demands. JADE is a noun referring to a green gemstone, but it can also be used to describe something that has been worn out or exhausted. Together, these two words provide an apt description of a tired old horse, which has been overworked and is now worn out.