How did the Jack Russell dog get its name?

Russell terriers were first bred by the Reverend John Russell, a parson and hunting enthusiast born in 1795.

https://en.wikipedia.org/wiki/Jack_Russel_Terrier

John ("Jack") Russell (December 12, 1795 – April 28, 1883), "The Sporting Parson" was an enthusiastic hunter and dog breeder as well as an ordained minister. He was reputed to be a man who enjoyed good living.

Born in Dartmouth, England, The Reverend Mr. Russell was educated at Blundell's School and Oxford University, and it was there, tradition has it, that he spotted a little white terrier bitch with dark tan spots over her eyes, ears and at the tip of her tail, who was owned by a milkman. Mr. Russell bought the bitch on the spot, and this girl, called 'Trump', became the foundation bitch of a line of fox hunting terriers that would eventually come to be known as Jack Russells

"Russell terriers were first bred by the Reverend Mr. John Russell, a parson and hunting enthusiast born in 1795. In his last year of university at Oxford he bought a small white and tan terrier bitch called Trump from the milk man. Trump was purchased based upon appearance alone."

"The first split between the many types of Russell terriers available today may have occurred early in their history with dogs being sold by the sister of John Russell's kennel man. These she described as "Jack Russells" but they may not have been part of the line of terriers developed by John Russell."


https://en.wikipedia.org/wiki/Jack_Russel_Terrier

Here's what the Jack Russell Terrier Club of America has to say:

The Jack Russell Terrier takes it name from the Reverend John Russell who bred one of the finest strains of terriers for working fox in Devonshire, England in the mid-to-late 1800's. Rev. Russell (1795-1883), apart from his church activities, had a passion for fox hunting and the breeding of fox hunting dogs; he is also said to be a rather flamboyant character, probably accounting for his strain of terrier's notability and the name of our terrier today. His first terrier, the immortal TRUMP, is said to be the foundation of John Russell's strain of working terriers.

http://www.terrier.com/breed/history.php3