misfortune, sin, evil
孼 centers on a child born outside the main family line, carrying a sense of misfortune, sin, or evil. The meanings extend from the idea of an illegitimate or aberrant offspring to broader calamity and wrongdoing.
孼 is traditionally explained as combining 辥 (a phonetic element related to cutting or separating) with 子 (child), suggesting a child cut off from the main line, hence illegitimate or bringing misfortune. The exact historical development is uncertain.
The top 辥 looks like a tangled, thorny thicket, and the bottom 子 is a child. Imagine a child tangled in misfortune and sin, born outside the proper family line.
For ゲツ, picture a child getting tangled in the thicket and letting out a frustrated 'getsu!' sound: getsu -> ゲツ, as the child struggles with the misfortune.