出典:Wiktionary
From earlier *interus (whence also interior), from Proto-Indo-European *h₁énteros (“inner, what is inside”). Cognates include Sanskrit अन्तर (ántara, “interior”) and Ancient Greek ἔντερον (énteron, “intestine, bowel”), Northern Kurdish hindirr (inside), Persian اندرون (“inner”).
The change from instrumental/ablative and accusative to accusative only is caused by *-teros used adverbially.
intrā (+ accusative)
参内する
to go between two parties―mediate between two parties―interpose between two parties―intervene between two parties―act as go-between―act as mediator
go within
to keep the matter a strict secret
はって中に入る
内臓で
to pierce something
uj
the inner part of something