懂中文 Dong Chinese

瞭如指掌

liǎo​rú​zhǐ​zhǎng

to know sth like the back of one's hand

Definitions

了如指掌 (瞭如指掌) 

liǎo​rú​zhǐ​zhǎng

variant of 了如指掌[liǎo​rú​zhǐ​zhǎng]

Sources