thirty
丗 represents the number thirty, a specific numeric value.
丗 is an older form of 世, originally derived from a pictograph of three ten-like shapes joined together to represent thirty. The modern form 世 is more common, while 丗 retains the extra stroke.
The shape resembles three tens (十) stacked or combined, visually suggesting thirty.
For セイ, imagine saying 'say' thirty times: 'say' -> セイ, and the repetition reinforces the number thirty.