لثفة doesn't appear in Hans Wehr's Modern Standard Arabic dictionary, but Lane's Arabic-English Lexicon has an entry for it. Select the entry below for the full classical Arabic definition.

• لُثْفَةٌ

  لُثْفَةٌ ذ The changing, in pronunciation, س into ث, or ر into غ or ل, (S, K, Msb,) and the like: (Msb:) or, one letter into another. (Az, in Msb, K.) Also, A word mispronounced; as when a word is said to be لُغَةٌ أَوْ …

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