ten thousand, 10,000
萬 centers on the large number ten thousand, often used to express a vast or countless amount.
萬 originally depicted a scorpion, which was later borrowed to represent the number ten thousand. The modern form retains the scorpion-like shape with the grass top 艹 and the body 禺.
The top 艹 looks like grass, and the bottom 禺 resembles a scorpion. Imagine a vast field of grass where ten thousand scorpions are hiding.
For マン, picture a man standing in awe before ten thousand scorpions: man -> マン, and the sheer number is overwhelming.