The word "yakkiest" is spelled with a double "k" because the sound it represents is a voiced velar stop (IPA /k/). When this sound is followed by the vowel "i", the letter "k" is doubled to indicate that the sound should be pronounced with more emphasis. Additionally, the letters "y" and "ie" are used to represent the sounds /j/ and /i/, respectively. Therefore, the word is spelled phonetically as /ˈjæk.i.ɪst/.