truly

before 1000; Middle English treuli, Old English trēowlīce. See true, -ly

The English adverb truly is obviously composed of the adjective true and the suffix -ly, which is used to form adverbs as well as adjectives (such as friendly ). The Old English form is tréowlíce, which means “faithfully,” formed from tréowe “faithful, trustworthy” and the adverb suffix -líce, which becomes -ly in modern English.
Old English -líce ultimately derives from the Germanic noun līkom “body, form, appearance.” This Old English suffix means something like “having the form or appearance of,” and tréowlíce therefore means “appearing faithful, looking trustworthy.”
The Romance languages (such as Spanish, Italian, French) have chosen mind over body in forming adverbs. In Spanish, claro means “clear,” and its adverb is claramente; likewise, Italian chiaro and chiaramente. In Latin this would be a phrase composed of the noun mēns “mind” and the adjective clārus “clear”: clārā mente “with a clear mind.” Even today Spanish (and Portuguese) maintain mente in its original noun sense (and not as an adverb suffix) in a chain of adverbs: thus “distinctly and clearly” is distinta y claramente in Spanish ( distinta e claramente in Portuguese), a usage that also existed in Old Italian and Provençal.