A devotee is also called a muni. The word muni means "thoughtful." A devotee is as thoughtful as a nondevotee is speculative. The nondevotee's speculation is impure, but a devotee's thoughts are pure. Lord Kapila and Sukadeva Gosvami are also called muni, and Vyasadeva is addressed as Mahamuni. A devotee is addressed as muni, or thoughtful, when he purely understands the Supreme Personality of Godhead.

From Srila Prabhupada's purport to SB 4.24.59