The word "duckiest" is spelled with a "k" because it derives from the word "duck" and not "duce". The IPA phonetic transcription for "duckiest" is /ˈdʌkiəst/. The "k" sound in the middle of the word is represented by the letter "k" in the spelling. The vowel sound in the first syllable is represented by the letter "u" and the second syllable is represented by the letters "ie". Overall, "duckiest" is spelled according to the standard English spelling conventions for words derived from "duck".
The word "duckiest" is derived from the noun "duck". The etymology of "duck" traces back to Old English "duce", which originally referred to a diving waterbird known for its quick movements and ability to plunge into the water. Over time, the word "duce" shifted to "duke" and "duck", and the latter term came to specifically denote the female waterbird. The adjective "ducky" was formed by adding the suffix "-y" to "duck", indicating something cute, pleasant, or good-natured. From "ducky", the superlative form "duckiest" can be derived, meaning the most cute, pleasant, or good-natured.