Three days without reading and one's speech becomes flavorless, one's face repulsive.
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history
From Han Yu's 《送穷文》 (Tang dynasty, 'Farewell to Poverty'): 凡所以使吾面目可憎，语言无味者，皆子之志也 - 'Everything that makes my face repulsive and my words flavorless is your doing' (addressing the spirit of poverty). The famous later quote 三日不读书，便觉言语无味，面目可憎 ('three days without reading and one's face becomes repulsive') is attributed to the Song poet 黄庭坚 (Huang Tingjian) and made the idiom a literary commonplace for the importance of self-cultivation.
register
Strong literary register; uses 憎 (zēng, 'to detest'). The face is so off-putting it provokes hatred - much stronger than English 'unattractive'.
