IPA: /ˈmʌs.kli.ər/
KK: /ˈmʌs.kli.ɚ/
Having well-defined muscles or a strong physique; more muscular than someone or something else.
He looked even musclier after months of weight training.
Comparative: more muscly
Superlative: most muscly
Musclier is derived from "muscle" (from Latin "musculus", meaning little mouse, referring to the shape of muscles) and the suffix "-ier" (meaning one who or characterized by). The word "musclier" refers to someone who is characterized by having more muscle or being more muscular.
Think of 'muscle' as referring to the shape of a little mouse, and '-ier' indicating someone who has more of that quality — that's why musclier means someone who is more muscular.