The adverb 'stiffly' is built directly upon its Old English adjective root stīf, which meant firm, rigid, or powerful. This Germanic root is traced back to the Proto-Indo-European base steyp-, meaning to cram, stuff, or pack tightly, a concept inherently linked to becoming rigid or dense. The adverbial suffix -līce, originating from a noun meaning 'body' or 'form', evolved into a standard grammatical tool for creating adverbs in English. Thus, 'stiffly' literally means 'in a rigid form or manner', a sense which extended metaphorically to describe formal, unyielding, or strained behavior and movement.

Structure: stiff (rigid) + ly (adverb suffix) - stiff: From Old English stīf (rigid, unbending). Its functional role is the core adjective root conveying the meaning of rigidity. - ly: From Old English -līce (forming adverbs from adjectives). Its functional role is a derivational suffix that changes the adjective into an adverb.