Under the radar - doing something without other people noticing. Example - "We're keeping our relationship under the radar because we don't want her father to know about us yet"
Formidable - inspiring fear or respect through being impressively large, powerful, intense, or capable. Example - "He had to fight a formidable adversary"
Vested interest - a personal reason for involvement in an undertaking or situation, especially an expectation of financial or other gain. Example - "Banks have a vested interest in the growth of their customers"
Doubting Thomas - a person who is skeptical and refuses to believe something without proof. Example - "I was a real doubting Thomas. I didn't believe the stuff would work"
To pick up/take up the slack - to do the work that someone else has stopped doing but still needs to be done. Example - "If Sue gets a job, Mark will have to take up the slack at home"
Pandora's box - a source of extensive but unforeseen troubles or problems. Example - "The senate investigation turned out to be a Pandora's box for the administration"
Can of worms - a situation that causes a lot of problems for you when you start to deal with it. Example - "When you brought that up, you opened a whole new can of worms"
To press/push one's luck - to try too hard to get a particular result and risk losing what you have achieved. Example - "She's agreed to help on Saturday, but I think I'd be pushing my luck if I asked her to be here the whole weekend"
To lead somebody up the garden path - to deceive someone. Example - "We were led up the garden path about the cost of the building work - it turned out really expensive."
To worship the ground somebody walks on - to love and admire someone very much. Example - "She always admired her Linguistics professor. In fact, she worshiped the ground he walked on"
To ream someone out - to tell someone off severely because you strongly disapprove of their behaviour. Example - "The boss reamed them out for sleeping on the job"
Against one's better judgement - if you do something against your better judgement, you do it although you think it is wrong. Example - "I lent him the money against my better judgement"
Whirlpool - a situation in which you lose control of what is happening, usually in an unpleasant way. Example - "She endured a whirlpool of conflicting emotions last year"
Watch somebody or something with an eagle eye - to watch someone or something very closely. Example - "The teacher was watching the children with an eagle eye, making sure they behaved themselves"
Bonhomie /ˈbɒnəmiː/ - a feeling of friendliness among a group of people. Example - "I was amazed by the bonhomie of strangers singing together around the campfire last night"
To run the gauntlet - to have to deal with a lot of people who are criticizing or attacking you. Example - "Every day they had to run the gauntlet of hostile journalists on their way to school"
To end something on a high note - to finish, complete or leave (something) at a successful, impressive or climactic point. Example - "I know we've had quite a few losses this season, but let's win this game and end on a high note"
Tautological - characterized by repetition and excessive wordiness. Example - "His works cover nearly 40 volumes, often obscure, often tautological, and with no great distinction of style"
Pecking order - the way in which people or things in a group or organization are placed in a series of levels with different importance or status, a social hierarchy. Example - "As an assistant manager, he was pretty low in the company's pecking order."
Slack-jawed - a person with the mouth in an open position and the jaw hanging loosely, especially as indicating bewilderment or astonishment. Example - "She left us all slack-jawed with her news"