The present participle of 'environ'; the act of surrounding or encircling something.
From 'environ' (Old French 'environner') plus the suffix '-ing,' which forms the present participle or gerund of verbs.
'Environing' appears in older literature to mean 'surrounding'—notice how the 'en-' prefix means 'to cause' or 'to make.' So 'environing' literally means 'making surroundings around something.' The prefix is doing all the work!
Complete word intelligence in one call. Free tier — 50 lookups/day.