ISTG is an acronym that stands for "I swear to God". It is used to express emotional sincerity, annoyance, or surprise in online messaging and texting conversations. ISTG can be written in both uppercase and lowercase letters, and they are interchangeable. The acronym is commonly used by teenagers and young adults in social media platforms and messaging apps. Here are some examples of how ISTG can be used:

• "ISTG, I never would have flirted with him if I knew you were together."
• "Please forgive me, Ill never do it again, istg."
• "Ill do my homework right after the party, ISTG!"
• "If I dont get the classes I want, istg Im transferring schools."
• "ISTG, if you dont turn down your music, youre grounded."
• "Paul asked to borrow money again. ISTG that guy is always broke."

It is important to note that ISTG is a very casual acronym and should be avoided in professional or business scenarios.