Almost always used in the compound 厕所. More polite alternatives: 洗手间 or 卫生间.
register
Standard reading is cè. In some northern dialects 厕 is read sì, but only inside the word 茅厕 (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 the traditional form's broader 广 shelter.
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 (side), 测 (measure), 厕 all share both phonetic and tone.
