I always thought constipation should have been 便強
The polar opposite is a little clearer: 下痢
I think it’s a case of the original ancient character meaning something like “smooth movement” and then that got applied to other related concepts.
Just be sure to study grammar outside WK because WK alone is not enough for you to read and understand the Japanese content.
Ahh, so that’s why it’s still used in referring to flights, too.