threat, long ages
刧 centers on a forceful threat or coercion, and by extension a long, oppressive period of time (an age or kalpa). The two meanings connect through the idea of enduring a prolonged, threatening situation.
刧 combines 去 (go away, leave) with 刀 (knife, sword), suggesting a threat with a weapon that forces someone to leave or endure. The 'long ages' meaning likely comes from the Buddhist concept of a kalpa, an immensely long period, possibly through phonetic loan or semantic extension of enduring a threat.
A knife (刀) is held up to make someone go away (去): a clear threat. Imagine being forced to endure a long, threatening wait under that knife.
For コウ, imagine a threatening figure holding a knife and shouting 'COW!' to scare you away: COW -> コウ, and the threat makes you flee.