(informal) you can say that again!

never say never

not to know how right one is (said when someone who has uttered a statement doesn't know that this statement is even truer than they thought, for example because there is another dimension to it that they weren't aware of)

(informal) tell me about it

(informal) tell me about it!

Alternative form of c'est le moins qu'on puisse dire

(informal) to say excessive things, to say things that are unsavory, harsh, shocking, unpleasant, nasty, ugly

if you know what I mean

not need to be told twice

(informal) before you can say Jack Robinson, before you can say knife, faster than a minnow can swim a dipper, in two shakes, in a flash, in a jiffy