مُثَلِّثٌ

A calumniator, or slanderer, of his brother [or fellow] to his prince; because he destroys three; namely, himself and his brother and his prince: (Sh, T, M, * K:) as also ↓ مُثْلِثٌ; (K;) or thus according to Aboo-'Owáneh. (Sh, T.)

See also ثِلْثٌ, last sentence:

and see ثَلُوثٌ.