harpoon, gaff
銛 centers on a sharp metal tool for catching fish: a harpoon or gaff. Both meanings share the idea of a pointed metal implement used to pierce and secure.
銛 combines 金 (metal) with 舌, which likely contributes the sound and may relate to a piercing or pointed shape. The exact historical role of 舌 is uncertain.
The left side is metal 金, and the right side 舌 looks like a tongue with a barbed tip. Imagine a metal tongue that shoots out to pierce fish: a harpoon.
For セン, picture a harpoon with a sharp metal tip that costs one hundred (百, ひゃく) sen (銭, せん) coins: sen -> セン, and the metal tip is worth every sen.