IPA: //ʌnˈhɛlθfəl//
KK: /ʌnˈhɛlθfəl/
Causing or likely to cause bad health; not good for your body.
Eating too many unhealthful snacks can lead to weight gain.
Unhealthful → It is formed from "un-" (meaning not) and "healthful" (from "health" meaning the state of being free from illness and "-ful" meaning full of). The word "unhealthful" means not full of health or detrimental to health.
Think of 'not' ('un-') being added to 'full of health' ('healthful') — that's why unhealthful means something that is not good for your health.