A portmanteau of (Mountain Dew and Moonshine). Its a concoction originating in the Southern United States of the popular soda Mountain Dew and moonshine (crudely distilled Ethanol Alcohol). It should be noted that Dewshine has been made with racing fuel containing methanol with deadly consequences.
Papa Jethro: Dammit Jethro, how'd you wreck my car?
Jethro Jr: I went to Billy Bobs and we got all hopped up on Dewshine!
Papa Jethro: Dagnabbit!