Atibṛddha (Atibriddha) is one of the one hundred and eight epithets of Śiva Mahādeva (Siva Mahadeva). The prefix ati serves to suggest ‘much’ or ‘more’. The Sanskrit root bṛdh (bridh) means ‘to grow’. The lexical meaning of the word bṛddha (briddha) is ‘old’ or ‘ancient’. However, when the epithet is applied to Śiva, it does not necessarily signify Him as an old person in a materialistic sense. One prospering in various qualities, knowledge and wealth can also be called bṛddha in terms of his affluence in these things. Nīlakaṇṭha (Nilakantha), the famous critic, cites a popular verse in this context –

viprāṇāṃ jñānato jyaiśṭhaṃ kṣatriyāṇāṃ tu śauryataḥ
vaiśyāṇāṃ dhanabāhulyācchūdrāṇāṃ vayasā’dhikaḥ.

It means,a brāhmaṅa (brahmana) becomes known as bṛddha as his knowledge grows, a kṣatriya (kshatriya) as his prowess, a vaiśya (vaisya) as his wealth, and a śūdra (sudra) as his age. As Śiva Mahādeva is considered the oldest deity of all, he is definitely more ancient than any sign of existence in the universe. Besides, he is also the fountainhead of extreme knowledge, wealth and other qualities. These are the reasons behind the epithet of Atibṛddha being bestowed on Him. Nīlakaṇṭha’s explanation of this epithet also runs along this line –

guṇairjñānaiśvaryādibhiradhikatvena bṛddha ityarthaḥ na tu balīpalitādinā