Where does “ความแจ้ง” come from?

ความแจ้ง (Thai) comes from Thai ความ, from Proto-Tai ɣwaːmᴬ — matter.

Ancestry of “ความแจ้ง”, step by step

ความแจ้ง traces back through two separate lines of ancestry.

via Thai ความ

StepLanguageWordMeaning
1Thaiความaffair; matter; content; essence; subject;...
2Proto-Taiɣwaːmᴬmatter

via Thai แจ้ง

StepLanguageWordMeaning
1Thaiแจ้งfree from darkness, filled with light, or full of light: clear, bright, light, sunny, shiny, shining, etc
Every word from Proto-Tai ɣwaːmᴬEvery word from Thai ความ
ความแจ้ง — Etymology, Origin & Word History | Etymology Explorer