沐 originally meant specifically 'to wash one's hair' (per 《
说文解字》:
沐,
濯发也) — distinct from
浴 (yù, 'wash the body'),
沬 (mèi, 'wash the face'), and
洗 (xǐ, 'wash hands/feet'). Ancient Chinese had a precise word for washing each body part. Today these distinctions have faded;
沐 mostly survives in compounds like
沐浴 (mùyù, 'bathe') and the literary phrase
沐恩 (mù'ēn, 'to receive grace').