- A bird in the hand is worth two in the bush
- A bolt from the blue
- A bun in the oven
- A bunch of fives
- A chain is only as strong as its weakest link
- A change is as good as a rest
- A country mile
- A countenance more in sorrow than in anger
- A Daniel come to judgement
- A diamond in the rough
- A diamond is forever
- A different kettle of fish
- A dish fit for the gods
- A dog is a man's best friend
- A drop in the bucket
- A drop in the ocean
- A fate worse than death
- A feather in one's cap
- A fish rots from the head down
- A fish out of water
- A fly in the ointment
- A fool and his money are soon parted
- A fool's paradise
- A foot in the door
- A foregone conclusion
- A friend in need is a friend indeed
- A golden key can open any door
- A good beginning makes a good ending
- A good hiding
- A good man is hard to find
- A hard man is good to find
- A hiding to nothing
- A horse, a horse, my kingdom for a horse
- A house is not a home
- A house divided against itself cannot stand
- A journey of a thousand miles begins with a single step
- A king's ransom
- A knight in shining armour
- À la carte
- À la mode
- A legend in one's own lifetime
- A leopard cannot change its spots
- A little bird told me
- A little knowledge is a dangerous thing
- A little of what you fancy does you good
- A load of cobblers
- A load of codswallop
- A long chalk
- A man after my own heart
- A man who is his own lawyer has a fool for a client
- A millstone around one's neck
- A ministering angel shall my sister be
- A miss is as good as a mile
- A nation of shopkeepers
- A nest of vipers
- A no-brainer
- A nod is as good as a wink
- A norange
- A penny for the guy
- A penny for your thoughts
- A penny saved is a penny earned
- A picture is worth a thousand words
- A piece of the action
- A pig in a poke
- A place for everything and everything in its place
- A plague on both your houses
- A priori
- A problem shared is a problem halved
- A red rag to a bull
- A riddle wrapped up in an enigma
- A rising tide lifts all boats
- a rod for your own back
- A rolling stone gathers no moss
- A rose by any other name would smell as sweet
- A rose is a rose is a rose
- A safe pair of hands
- A sea change
- A skeleton in the closet
- A sledgehammer to crack a nut
- A shadow of a doubt
- A shot in the arm
- A sight for sore eyes
- A sorry sight
- A spanner in the works
- A stitch in time saves nine
- A stone's throw
- A thing of beauty is a joy forever
- A tinker's damn
- A turn up for the books
- A watched pot never boils
- A wet blanket
- A wolf in sheep's clothing
- A woman needs a man like a fish needs a bicycle
- A woman's place is in the home
- A word in edgeways
- A word in your shell-like
- A worm will turn
- A young man's fancy
- A1 at Lloyds
- Abandon all hope ye who enter here
- Abide with me
- About face
- About turn
- Above board
- Above my pay grade
- Abracadabra
- Absence makes the heart grow fonder
- Absent-minded professor
- Absent without leave
- Absolute power corrupts absolutely
- Accident of birth
- Accidentally on purpose
- Accidents will happen
- According to Hoyle
- Ace in the hole
- Achilles' heel
- Acid test
- Acronyms
- Across the board
- Across the pond
- Act of God
- Act the giddy goat
- Action man
- Actions speak louder than words
- Adam's ale
- Adversity makes strange bedfellows
- After the fact
- Aga saga
- Against the clock
- Against the grain
- Age before beauty
- Age cannot wither her, nor custom stale her infinite variety
- Agree to disagree
- Aid and abet
- Aide-mémoire
- Air kiss
- Air quotes
- Al fresco
- Alas, poor Yorick! I knew him, Horatio
- Alike as two peas in a pod
- Alive and kicking
- All agog
- All and sundry
- All at sea
- All fingers and thumbs
- All Greek to me
- All in all
- All intents and purposes
- All of a sudden
- All present and correct
- All publicity is good publicity
- All singing, all dancing
- All systems go
- All that glitters is not gold / All that glisters is not gold
- All the tea in China
- All things come to he who waits
- All things must pass
- All you can eat
- All's fair in love and war
- All work and no play makes Jack a dull boy
- Alter ego
- Amber nectar
- An accident waiting to happen
- An Aladdin's cave
- An albatross around one's neck
- An apple a day keeps the doctor away
- An arm and a leg
- An axe to grind
- An Englishman's home is his castle
- An eye for an eye, a tooth for a tooth
- An ill wind
- An offer he can't refuse
- An Oxford scholar
- Anchors aweigh
- Angry young man
- Ankle biter
- Annus horribilis
- Another day, another dollar
- Another think coming
- Any port in a storm
- apple never falls far from the tree
- apple of my eye
- Apple pie order
- April fool
- ear candy
- Arms akimbo
- Artificial intelligence
- Ars longa, vita brevis
- Arty-farty
- Asap – As soon as possible
- As X as Y
- As alike as two peas in a pod
- As bald as a coot
- As black as Newgate's knocker
- As bold as brass
- As brown as a berry
- As busy as a bee
- As busy as a one-armed paperhanger
- As cold as any stone
- As cool as a cucumber
- As cute as a bug's ear
- As daft as a brush
- As dead as a dodo
- As dead as a doornail
- As different as chalk and cheese
- As easy as pie
- As easy as taking candy from a baby
- As fast as greased lightning
- As fine as frog's hair
- As fit as a butcher's dog
- As fit as a fiddle
- As good as gold
- As good luck would have it
- As happy as…
- As happy as a clam
- As happy as a sandboy
- As happy as Larry
- As high as a kite
- As keen as mustard
- As mad as a hatter
- As mad as a March hare
- As nice as ninepence
- As old as Methuselah
- As old as the hills
- As pleased as Punch
- As pure as the driven snow
- As queer as a nine bob note
- As safe as houses
- As sick as a parrot
- As snug as a bug in a rug
- As straight as a die
- As the crow flies
- As thick as thieves
- As thick as two short planks
- As white as snow
- Ashes to ashes dust to dust
- Ask a silly question and you'll get a silly answer
- for whom the bell tolls
- asta la vista, baby
- At loggerheads
- At one fell swoop
- At one's beck and call
- At one's wit's end
- At sixes and sevens
- Attack is the best form of defence
- Augur well
- Auld lang syne
- Away with the fairies