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.
GFG is an acronym that means good fucking game, and it is used sarcastically in online gaming. A more sincere version is GG, or good game. GFG is also used to mean gift from God.
In the online dating world, HWP means height weight proportionate. It's used in place of specific numbers, and some claim it should be interpreted to mean that one is higher on the weight scale than…
NMU is an acronym used to mean Not much, you. It's used in reply to the question, "ºÚÁÏÍø's up?" or "ºÚÁÏÍø are you doing?," often expressed as WYD (what you doing).