Antonyms of adj overzealous

1 sense of overzealous

Sense 1
fanatic, fanatical, overzealous, rabid -- (marked by excessive enthusiasm for and intense devotion to a cause or idea; "rabid isolationist")

INDIRECT (VIA passionate) -> passionless -- (not passionate; "passionless observation of human nature")

Similarity of adj overzealous

1 sense of overzealous

Sense 1
fanatic, fanatical, overzealous, rabid -- (marked by excessive enthusiasm for and intense devotion to a cause or idea; "rabid isolationist")
       => passionate (vs. passionless) -- (having or expressing strong emotions)