The word 'sternly' is built upon the adjective 'stern', which originates from the Old English styrne, meaning severe, strict, or hard. This Old English term is of Germanic descent, likely related to the concept of rigidity or harshness. The adverbial suffix '-ly' is also of Old English origin (-līce), a common Germanic construction for converting adjectives into adverbs. Thus, the literal combination of 'stern' (strict) and '-ly' (in the manner of) directly yields the modern meaning of acting in a severe, serious, or uncompromising manner.

Structure: stern (strict) + ly (adverb suffix) - stern: From Old English styrne (severe, strict, hard), of Germanic origin. - ly: From Old English -līce (forming adverbs from adjectives), of Germanic origin.