A person who is addicted to Arrowhead Spring Water. Arrowheads are most commonly seen drinking out of a 1 gallon jug. This is thought to be because of the bottles' optimal price and capacity.
The term is a combination of 'Arrowhead' and 'head'(as in, crack'head', pot'head', etc)
Dude1: Dude, I saw your economics teacher carrying around a 1 gallon bottle of Arrowhead Spring Water!! Hahahahaah!
Dude2: I know dude! He is such an Arrowhead!
"ist" is the combination of 'is' and 'it'.
"ist" is similar to, or the mere opposite of, "it's", which is a combination of 'it' and 'is'.
Child: Hey Baby! This looks like a fun game!
Baby: Really? What ist?