expression, noun-modifying (irregular)
well aware of; know thoroughly; familiar with (and thus feel no need for restraint)
Used when someone knows a place, person, or situation so well that they act freely and without hesitation, often implying a sense of ease or lack of formality.
He acted as if he were in his own home, completely at ease.
勝手知ったる道だから、迷わずに行ける。
Since it's a road I know like the back of my hand, I can go without getting lost.