know smb. as one knows his ten fingers

(know smb. (или smth.) as one knows his ten fingers (тж. know smb. или smth. like the back или palm of one's hand, уст. as well as a beggar knows his dish))

прекрасно знать кого-л. (или что-л.); ≈ знать кого-л. (или что-л.) как свои пять пальцев

I know thou knowest every place by the river's side as well as... the beggar knows his dish. (W. Scott, ‘The Fortunes of Nigel’, ch. XXI) — Ты, я знаю, изучил все места на Темзе... как нищий свою суму.

Martin knows that place like the palm of his hand. (C. P. Snow, ‘The Affair’, ch. 11) — Мартин знает колледж как свои пять пальцев.

He knew his assets and his liabilities, as he knew his ten fingers. (Th. Dreiser, ‘The Financier’, ch. XXX) — Он прекрасно знал состояние своего актива и пассива.

I've lived in New York all my life, and I know Central Park like the back of my hand. (J. Salinger, ‘The Catcher in the Rye’, ch. XX) — Всю жизнь я прожил в Нью-Йорке и знаю Центральный парк вдоль и поперек.