The first recording of the word "dad" was in the 16th century. However it is most likely to have originated much earlier, from syllables commonly mouthed by toddlers such as "dada."
According to the Online Etymology Dictionary, "Dad" and "daddy" originate from children's speech and may have prehistoric roots that would make it a linguistic object universal to many language groups. The word has similarly sounding equivalents in several languages, including the Welsh "taid," the Irish "daid," "tete" in Lithuanian, "tata" in Greek, and "tatah" in Sanskrit, which all have the same meaning.
Since the word "dad" in its various forms is such a common occurrence across languages, this supports speculation that it may have extremely old and possibly prehistoric roots. However, concrete evidence for the origin of the word is scarce.
The syllables "ta," "da," and "la" are known as dental consonants, which are made with the tongue held against the teeth. According to the National Institute on Deafness and Other Communication Disorders, they are some of the easiest sounds for toddlers to voice once their upper teeth settle in. Similarly to "mom," which is related to the nasal sounds that can be created when the baby's lips are pressed to a mother's breast, "dad" is most likely to have originated from the earliest sounds typically made by toddlers.