What is "untapable"

Untapable (foaled 13 February 2011) is an American Thoroughbred racehorse. She showed promise as a two-year-old in 2013 when she won two of her four races including the Pocahontas Stakes and was placed in the Starlet Stakes. In 2014 she emerged as a top-class racehorse, winning the Rachel Alexandra Stakes, Fair Grounds Oaks, Kentucky Oaks, Mother Goose Stakes, Cotillion Handicap and Breeders' Cup Distaff and was named American Champion Three-Year-Old Filly.