IPA: /ˌʌn.əˈθlɛt.ɪk/
KK: /ʌnəˈθlɛtɪk/
Not good at sports or physical activities; lacking physical strength or skill.
He felt unathletic compared to his friends who played sports every weekend.
Unathletic → It is formed from "un-" (meaning not) and "athletic" (from Greek "athletikos", meaning pertaining to athletes or athletic activities). The word "unathletic" means not athletic or lacking physical fitness or prowess.
Think of 'not' ('un-') being added to 'athletic', which refers to physical fitness — so unathletic means lacking in physical fitness.