moisten, wet, soak, touch
沾 centers on liquid making contact: moistening, wetting, or soaking something. The 'touch' sense is a figurative extension of liquid reaching a surface.
沾 combines 氵 (water) with 占, which likely contributes the sound. The character expresses the idea of water making contact and moistening.
Water 氵 touches and spreads over 占, like a drop of water landing on a surface and soaking in.
For セン, imagine a cent (coin) dropped into water, sending ripples that moisten everything: cent -> セン.