The Brief: ISTG is an acronym for "I swear to God."

DEEP DIVE
ISTG is shorthand for “I swear to God.” As Dictionary.com explains, the acronym and phrase is used to convey extreme seriousness, exasperation, or surprise, and is usually used outside of a serious religious context.
Istg Iโm praying for anybody thatโs dealing with one of them ๐คฃ๐ฅด https://t.co/NIFb2eXLfG
— LLA๐ (@tweetdomiii_) March 19, 2021
skk writers make me so violent istg pic.twitter.com/9k2bBRpzC5
— ๐; rii (@dazaivsan) March 19, 2021