A Bad Apple, A Bad Egg, One's Bread and Butter, A Piece of Cake

A Bad Apple
A "bad apple" is someone who might not be in trouble with the law (as is the case for "a bad egg"), but is a person who has a bad attitude (often about work), who is a negative influence in a team, and who is generally not pleasant to work with or be around.
Ex.: "That guy we just hired? I think he's a bad apple. He snaps at co-workers; he's becoming a real distraction." The new employee is creating negativity around him.


A Bad Egg
Figuratively, a bad egg is someone who is often involved in trouble, such as petty crime, vandalism, and so forth.
Ex.: "I don't want you hanging around with that boy! He's a bad egg. He'll get you arrested someday!" Here, "that boy" is well known as a delinquent and is involved with shady things (though not necessarily on a serious level... yet).


The Big Cheese
In an organization, the big cheese is the person in charge of a particular branch. Big cheeses, as a group, are synonymous with Very Important Persons (V.I.P.s).
Ex.: "No, I don't want to speak to his assistant. I want to talk to the big cheese as soon as possible." Here, "the big cheese" is the only person with the actual authority to make things happen.


Filling the Void
In food idioms, "the void" is the emptiness of the stomach, both literal and the "feeling" of hunger. Food that "fills the void" satisfies raw hunger - but does nothing else. It is not necessarily tasty, but at the very least, does not taste awful enough to be outright rejected.
Ex.: "So how was the meal you had at that restaurant?" "It filled the void." "...That's not much of a compliment!" <- For an expensive restaurant meal to only "fill the void" would be taken as an understated insult towards the restaurant.


One's Bread and Butter
Idiomatically speaking, a person's, or an organization's, bread and butter is the core or heart of that person or group's activities.
Ex.: "Microsoft's Windows operating system is its bread and butter. Even so, Microsoft has expanded into the video game console market, where it competes with Sony and Nintendo." Here, Windows is Microsoft's main business activity, while video games might be considered "gravy," figuratively speaking.

A Piece of Cake
A task that is "like eating a piece of cake" is a task that is easy. Cake, while fattening, is easy to digest. Therefore, doing a task like eating cake, is a task that is easy. Thus, the idiom, "a piece of cake," was born.
Ex.: "Driving a car? That's a piece of cake! Flying a plane? Now that's challenging." Something that is a figurative piece of cake, is easy.


Being "Out To Lunch"
A person whose mind is "out to lunch" is a person whose critical faculties and logical thinking centers are on vacation, not functioning, and so on. That is, the person who is "out to lunch" is irrational, absent-minded, unfocused, or (to use a non-scientific term), crazy.
Ex.: "Bill's totally out to lunch today. I heard his girlfriend dumped him... he'd better snap out of it, or the boss is going to be on his back about it." Bill cannot focus or concentrate and is doing poorly in his job.


Making One's Mouth Water
If the smell or sight of food is making my mouth water, it is making me hungry. My mouth is generating saliva in anticipation of eating food. Used figuratively, this is an idiom for anticipation in general.
Ex.: "My mouth's watering at the thought of buying the new Google Android phone. I can't wait!" This expresses eager excitement. Does a cell phone make your mouth water?

➤ Read more idioms and phrases, English grammar and vocabulary at Basic English Grammar And Vocabulary .