耳塞 (ěr sāi) literally means 'ear stopper' — 耳 (ěr) means 'ear', and 塞 (sāi) means 'to plug' or 'to block'. Together, they refer to a small device inserted into the ear canal to reduce noise, prevent water entry, or improve sleep. Earplugs are commonly made of foam, silicone, or wax, and are widely used in noisy environments like construction sites, concerts, or airplanes.

This word is neutral in register and appears in both spoken and written Chinese. It’s often used in health, safety, and daily life contexts — for example, when discussing hearing protection, travel preparation, or sleep hygiene. Unlike the verb 塞 (sāi), which can mean 'to stuff' in many contexts, 耳塞 functions exclusively as a noun meaning 'earplug' and is never used as a verb.