合不拢嘴

hé bù lǒng zuǐ

unable to conceal one's happiness, amazement, shock etc

grinning from ear to ear; mouth agape; gobsmacked

合

hé

variant of 合[he2]

不

bù

no; not so

拢

lǒng

to gather together; to collect

嘴

zuǐ

mouth