This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 冼 is uncertain. It combines 冫 (ice) and 先 (before/ahead), but the original meaning and component relationship are unclear.
With no clear meaning, picture ice (冫) ahead (先) as a cold front moving in, but remember this is just a visual hint.
For ショウ, imagine a cold show (ショウ) where ice (冫) is the star ahead (先) of the audience: show -> ショウ.