atonal (adj) – lacking tone, key, or musical center
BREAKDOWN: A- (without) + TON- (tone) + -AL (pertaining to)
— atony is a lack of muscle tone or energy
— atonic means lacking tone or energy
This word is built on the root TON-, which means tension, tone, sound, or thunder.
Common TON- words include astonish, detonate, monotonous, thunder, tone, and tune.