bamboo whisk for tea-making
筅 refers specifically to a bamboo whisk used in traditional tea-making, particularly for whisking matcha.
筅 combines 竹 (bamboo) with 先, which likely contributes the sound and may suggest the forward motion of whisking. The exact historical development is uncertain.
The bamboo top 竹 and the lower part 先 (ahead, tip) suggest bamboo tips used to whisk tea forward and back.
For セン, picture a thousand (千, せん) bamboo bristles on the whisk: 千 -> セン, and the whisk froths the tea.