"Shheee-it!"

One of the many delightful characters in the TV show The Wire is State Senator Clay Davis, played by Isiah Whitlock Jr. Davis had a characteristic expression—shheee-it!—that he used to punctuate his speech. Earlier in his career, Whitlock had played a character in the movie 25^th Hour who used the same expression.

Now I come across "Shheee-it!" in On the Road, spoken by a tenor saxophonist in San Francisco. Presumably it's not an invention of Kerouac's; perhaps he heard it under circumstances similar to those in the novel.

I wonder if the two usages have a common root.