مثم 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.

• مِثَمٌّ

  مِثَمٌّ مثم (like مِسَنٌّ, K [in the CK, erroneously, مُثِمّ, like مُسِنّ,]) One who pastures for him who has no pastor, (T, K,) or no pasturage, (TA,) and lends a beast or camel for riding or carrying, to him who has no…

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