big mouth	used as a criticism of someone when they say things thay should not
come clean about sth	tell the truth about sth that you have kept in secret
get/catch sb's drift	understand the basic meaning
get sth off your chest	talk to someone about sth that has been wottying you, so that you feel better about it
get the wrong end of the stick	understand sth completely wrong
give sb your word	promise sb to do sth
(hear sth) on/through the grapevine	the way in which information spreads quickly from one person to another through conversation
keep sb posted	regularly give someone information about sth they are interested in (how situation is changing etc.)
keep sth under your hat	keep sth secret
lay/put your cards on the table	tell people exactly what are you thinking or what you are intending to do
speak volumes	provide a lot of informiation, especially in an indirect way
tell tales	tell someone in authority about bad things that someone else had done, because you want to punish tm