Almost always used in the compound 厕所 (cèsuǒ). More polite alternatives: 洗手间 (xǐshǒujiān) or 卫生间 (wèishēngjiān).
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Standard reading is cè. In some northern dialects 厕 is read sì, but only inside the word 茅厕 (máosi, an outhouse or latrine). Everywhere in standard Mandarin the reading is cè.
厂 is a cliff-overhang radical and depicts a small lean-to structure — appropriate for an outhouse, which is what 厕 originally named. Indexing radical here, even though the simplified 厂 is much sparser than traditional 廁's 广.
则 supplies the sound, drifting from zé to cè (z- to c- is a regular old-Chinese alternation). 则 appears in many phono-semantic compounds — 侧 cè (side), 测 cè (measure), 厕 cè all share both phonetic and tone.
