Correct spelling for the English word "rapider" is [ɹˈapa͡ɪdə], [ɹˈapaɪdə], [ɹ_ˈa_p_aɪ_d_ə] (IPA phonetic alphabet).
The word rapider is formed by adding the suffix -er to the adjective rapid. The etymology of rapid can be traced back to the Latin word rapidus, which means swift, quick, or rapid. In English, rapid emerged in the late 16th century and was derived from the French word rapide with the same meaning. The suffix -er added to rapid forms the comparative form, indicating a higher degree of quickness or swiftness, hence rapider.