DHMU means “Don’t Hit Me Up“. It’s the opposite of HMU (Hit Me Up) (which is a request for a call-back or some other form of contact). In other words, DHMU is a rapid way to tell someone not to call (usually because their call would cause a disturbance).

ISTG is an acronym that means I swear to God. It can be used to convey extreme seriousness, exasperation, or surprise, and is usually used outside of a serious religious context.