deception
瞞 centers on hiding the truth: deception, trickery, or misleading someone by concealing what is real.
瞞 combines 目 (eye) with 㒼, which likely contributes the sound and may relate to covering or concealing. The character suggests hiding something from the eyes, i.e., deception.
The left side is an eye 目, and the right side 㒼 looks like a lid or cover over something. Picture someone covering your eyes so you cannot see the truth—that is deception.
For マン, imagine a man pulling a blindfold over your eyes: man -> マン, and his trick hides the truth.