that
厥 is a classical character that functions as a demonstrative, pointing to something previously mentioned or understood: 'that' or 'its'. Its meaning is grammatical rather than concrete.
厥 is an ancient character whose original pictographic or semantic basis is not clearly preserved in the modern form. It appears in classical Chinese as a possessive or demonstrative pronoun.
The outer 厂 can be seen as a pointing hand, and the inner 欮 as a complex object being indicated: 'that one'. Imagine a hand gesturing toward a specific item among many.
For ケツ, picture a person pointing at something and saying 'kets' as a sharp, decisive sound to indicate 'that one': kets -> ケツ.