From Wiktionary, the free dictionary
nkag (“to crawl, go beneath”) + siab (“liver”), literally “to enter the liver (metaphorical heart)”.
nkag siab
- to understand
- Koj puas nkag siab? ― Do you understand?
- Heimbach, Ernest E. (1979) White Hmong — English Dictionary[1], SEAP Publications, →ISBN, page 151.