The Mantra you quoted in the question is from Rig-Veda -Mandala 7 - Sukta 35 - The Rishi of this sukta is Vaisihtha MaitraVaruni . And Devata is Vishwadevas i.e. Various gods. These are Mantras for peace and bliss.
शं नो द्यावापृथिवी पूर्वहूतौ शमन्तरिक्षं दृशये नो अस्तु |
ओषधीर्वनिनो भवन्तु शं नो रजसस्पतिरस्तु जिष्णु: ||Rig-Veda
Early invoked, may Heaven and Earth be friendly, and Air's mid-region
good for us to look on. To us may Herbs and Forest-Trees be gracious,
gracious the Lord Victorious of the region.
The meaning of the word "Jishnu" translated by various authors are somewhat same .Jishnu (जिष्णु) = Victorious Lokpal or Lok Pati , Lord Victorious of the region, Victorious lord of the world etc.
Now lets see what other scriptures like Mahabharata and Puranas are saying about this word.
Starting from Mahabharata , the word Jishnu is used here for Arjuna. i.e. Jishnu is one of the ten names of Arjuna. Here in Virata Parva - Gau haran parva - Chapter 44 -Arjuna himself is telling his name Jishnu to uttara.
अहं दुरापो दुर्धर्षो दमन: पाकशासनि: |
जिष्णुर्नामास्मी विश्रुत : ||21||
I am known among human beings and celestials by the name of Jishnu,
because I am unapproachable and incapable of being kept down, and a
tamer of adversaries and son of the slayer of Paka.
Here Arjuna is saying that he is son of Indra , but the name is of Arjuna although mention of Indra.
In puranas like Garuda Purana
- Achar Kanda - Chapter 31 - Description of another form of Vishnu worship , the name Jishnu is used fro Lord Vishnu as one of his name uttered during worship.
जिष्णवे सर्वदेवानां सर्वगाय महात्मने ||
सर्वेशाय नमोनम: ||27||
salutation unto the great Jishnu the lord of all gods and present
everywhere. Salutation again and again unto the lord of all adored of
Brahma , Rudra and Indra.
In Vayu Purana - Chapter -25 - "Description of the obtainment of a boon from Shankara of Vishnu. " - Its said that Lord Vishnu (probably Maha Vishnu) created two of his forms , same as brothers one is Vishnu and another is "Jishnu" to protect Brahma from demons Madhu and Kaitabha.
गते तस्मिन्स्ततोSनन्त उद्गिर्य भ्रातरौ मुखात |
विष्णुं जिष्णुं च प्रोवाच ब्रह्माण भिरक्षताम|
मधुकैटभयोर्ज्ञ्नात्वा तयोरागमनं पुनः ||Vayu Purana 25.37 ||
When Brahma had gone , Lord Vishnu created two brothers - Vishnu and
Jishnu out of his mouth and instructed them - Both of you should
protect Brahma having known the arrival of Madhu and Kaitabha .
So again we can here see the association of the name Jishnu to Vishnu. And Vishnu created two brothers from his mouth (probably Nara-Narayana) .
So as you required the Puranas and Mahabharata meaning of word Jishnu is different than the meaning found in GujarAti Dictionary you have seen . Although one of the name of the Indra is jishnu , but the word is also used for Vishnu and Arjuna.
Here are the various meaning of the word from spokensanskrit