This is the 4889th most frequent Swedish word.
bini
"Bini" is not a recognized Swedish word. It may be a typo or slang.
'Bini' here is used colloquially to imply 'within' or 'into' in connection to a seat or similar objects.
Hon satte sig bini stolen.
She sat down into the chair.
'Bini' expresses movement into an enclosed space, such as the tunnel described.
Man måste krypa bini tunneln för att nå slutet.
One must crawl into the tunnel to reach the end.
Here, 'bini' denotes placement into an area typically described as enclosed or overarched.