coinciding, agreeing
脗 centers on things matching or coming together in agreement: coinciding, agreeing, or fitting harmoniously.
The exact historical development of 脗 is uncertain. It visually contains 月 (flesh) and 勿, but the component relationship is not clearly established.
The left side 月 (flesh) and the right side 勿 (which can suggest 'not') can be imagined as two pieces of flesh that must not be separated—they coincide and agree perfectly.
For モン, imagine two monks agreeing on a point of doctrine: monk -> モン, and their views coincide like the parts of 脗.