Does spitting a lot dehydrate you?

Short answer: Yes it does.

Saliva consists of mostly water (up to 98%). Most people generate around 0.75L to 1.5L of saliva per day, so by spitting often, you are losing water.

If you've ever spit a lot in a row, you will notice your mouth become more dry, as the body struggles to produce more saliva quickly. The overproduction of saliva is needed to combat the loss of saliva from spitting, which requires water. Do this too much, and you've started to dehydrate yourself.