The phrase "jonsing" is believed to originally derive from the Grateful Dead song "Casey Jones":
"Riding that train high on cocaine, Casey Jones you better watch your speed"
"Jonsing" means to be in a state of withdrawl and/or need that next fix. Originally for drugs, but it has come to generally mean any strong craving (for chocolate, sex, coffee, sugar etc.)
"I'm jonsing hard!"
"I quit smoking yesterday and I'm already jonsing"
"Jonsing for carbs"