number one, line, rod radical (no. 2)
丨 is a simple vertical stroke that represents a line or rod. It is also used as a radical component in other characters, and its meaning 'number one' comes from its use as a tally mark.
丨 is a pictograph of a vertical rod or line. Its use as a tally mark for 'one' is ancient, and it serves as a basic stroke in many characters.
Picture a single vertical rod standing upright. It looks like the number one written as a tally mark, and it forms the line radical used in characters like 中.
For コン, imagine a con artist drawing a single vertical line on a contract to trick someone: con -> コン, and the line is the mark of the scam.