acca.

The earliest Latin name of H was Classical Latin ha. The loss of /h/ in common speech before the end of the Republican period made this name indistinct from a (“the letter A”), driving its replacement by *acca much later. The OED sees *acca as a phonological normalisation of *ahha, a reinforcement of ha (compare the later development of michi, nichil). Sheldon instead sees here a fusion of ha + ka (“the letter K”). He notes that the practice of Latin grammarians was to separate the alphabet into vowels, "semivowels" (continuant consonants) and mutes. The list of mutes was B C D G H K P Q T, and in recitation of this sequence, ... ge ha ka pe..., the ha and ka could have accreted together. This would also explain the variant form *aca, found in Portuguese.