筷 is almost exclusively used in 筷子 (kuàizi, chopsticks). The measure word is 双 (shuāng, pair) or 根 (gēn, single stick). The 竹 radical on top indicates bamboo, the original material.
culture
Chopstick etiquette: never stick chopsticks upright in rice (it resembles funeral incense), and never point at people with them.
⺮ is the bamboo radical 竹 in compressed top form. It places 筷 in the bamboo family alongside 笔笑答篮, items traditionally made from bamboo, including the original chopsticks themselves.
快 supplies the sound kuài directly. Folk etymology says boatmen avoided the homophone 箸 (zhù = stop, bad luck for travel) and replaced it with the auspicious 筷, bamboo plus fast, wishing for quick journeys.
