1991 - Eric Lax - Woody Allen - To all appearances, theirs was a unique case of sibling amity.
Apple of discord
A subject of dissension
This expression refers to the Greek myth in which a golden apple inscribed for the fairest was contended for by the goddesses Hera, Athene and Aphrodite.
The apple of your eye
A person or thing of whom you are extremely fond and proud
Old English, the phrase referred to the pupil of the eye, considered to be a globular solid body
It came to be used as a symbol of something cherished and watched over.
Apples and oranges
(Of two people or things) irreconcilably or fundamentally different - North American
A rotten apple = A bad apple
A bad person in a group - typically one whose behaviour is likely to have a corrupting influence on the rest - Informal
Used to indicate that everything is in good order and there is nothing to worry about Australian informal
Apples and spice or apples and rice is Australian rhyming slang for nice.
Upset the apple cart
Wreck an advantageous project or disturb the status quo.
The use of a cart piled high with apples as a metaphor for a satisfactory but possibly precarious state of affairs is recorded in various expressions from the late 18th century onwards.
1996 - Business Age - The real test will be instability in China... Another Tiananmen Square could really upset the apple cart.
As American as apple pie
Typically American in character
1995 - New York Times Magazine - To reward people for something beyond merit is American as apple pie.
Apropos of nothing
Having no relevance to any previous discussion or situation
Seal of approval = Stamp of approval
An indication or statement that something is accepted or regarded favourably
This expression stems from the practice of putting a stamp (or formerly a seal) on official documents.
Tied to someone's apron strings
Too much under the influence and control of someone (especially used to suggest that a man is too much influenced by his mother).
A grey area
An ill-defined situation or field not readily conforming to a category or to an existing set of rules
In the 1960s, grey areas in British planning vocabulary referred to places that were not in as desperate a state as slums but which were in decline and in need of rebuilding.
2001 - Rough Guide to Travel Health - In theory, it should be a cinch to diagnose appendicitis, but in practice it's much more of a grey area.
A no-go area
An area which is dangerous or impossible to enter or to which entry is restricted or forbidden
As a noun, no-go was first used in the late 19th century in the sense of an impracticable situation. Its use in this phrase, with the sense of no-entry, is particularly associated with Northern Ireland in the 1970s.
1971 – Guardian - For journalists and others, the Bogside and Creggan estates are no-go areas with the IRA in total effective control.
Argue the toss
Dispute a decision or choice already made
Informal - chiefly British
The toss in this phrase is the tossing of a coin to decide an issue in a simple and unambiguous way according to the side of the coin visible when it lands.