A horse or pony of a light breed with a high-stepping trot, used in harness. The word is Middle English, and probably comes from Hackney in East London, where horses were pastured. The term originally denoted an ordinary riding horse (as opposed to a war horse or draught horse), especially one available for hire.
The word is the origin of hackneyed meaning ‘unoriginal and trite’, and ultimately of hack for a dull and uninspired journalist.
what you've never had you never miss often used in consolation; saying recorded from the early 20th century.