There is a certain nervous feeling one gets when getting to, oh, say, half a pack of cigarettes. Where is the next pack coming from? And when? And this nervousness kinda takes over and drives until the questions are answered and at LEAST one new pack has been secured. Until that time, you've got Pack Nerves.
It's the same kinda feeling like at a party and you misplace your pack. The full (or nearly full) pack of cigarettes is your Linus blanket.. Addiction makes it so.