Short for Hyperspeed Texting.
A pun on "High Test" (the gas).
Speeding extremely fast on a road while texting at the same time.
Dude! You're doing 150 in a 65 AND texting?? I'm gonna F*ing die because of your hytexting!
Daaaamn girl! You better stop hytexting before a cop pulls us over and takes your license away!