The Seventh Kanda of third Prapathaka of the Taittiriya Samhita of the Yajur Veda explains various Yagnas (Vedic fire-rituals), and in particular the third Prapathaka of this Khanda provides some of the hymns required for the Ashwamedha Yagna (horse ritual). Here is one of those hymns:
Hail! (To) meditation (I offer).
To that meditated upon hail!
Hail! (To) that which we meditate on (I offer).
To mind hail! Hail! (To) mind (I offer).
To Prajapati hail! To Ka hail! To Who hail!' To Whoever (katamásmai) hail!
To Aditi hail! To Aditi the great hail! To Aditi the gentle hail!
To Sarasvati hail! To Sarasvati the mighty hail! To Sarasvati, the purifying hail!
To Pusan hail! To Pusan guardian of travellers hail! To Pusan watcher of men hail!
To Tvastr hail! To Tvastr the seminal hail! To Tvastr the multiform hail!
To Visnu hail! To Visnu the Nikhuryapa hail! To Visnu the Nibhuyapa hail!
I'm interested in the last line, addressed to Vishnu. My question is, why is Vishnu called the Nikhuryapa and the Nibhuyapa? What do these words mean? Sanskrit dictionaries don't provide any help (they just say it's a name of Vishnu), and I don't think the name is in the Vishnu Sahasranama.
Does Sayana address this in his commentary on the Taittiriya Samhita? He usually provides (sometimes fanciful) etymologies for the names of gods.