His championship status became unchallengeable after defending the title for three consecutive times.

unchallengeable 是由前缀 un-（表示“不”）和形容词 challengeable（可挑战的）组成的形容词。challengeable 来自动词 challenge（挑战）加上后缀 -able（表示“能够……的”）。unchallengeable 合起来表示“不可挑战的、不可撼动的”。