A cunning ploy: Un stratagème astucieux
A cog in the machine: Un rouage dans la machine
A fug of grief: Une gueule de chagrin
A white lie: Un pieux mensonge
After a good deal of practice
And I hardly need add
Anew: À nouveau
As you see fit: Comme bon vous semble
At point blank: À bout portant
Attagirl/Attaboy: Bravo
Behold!
Bummer: Dommage
But of course
Calm down
Can I speak plainer?
Capital offense
Cheerfully: Gaiement
Don't get ahead of yourself: Ne t'emballe pas, Ne mets pas la charrue avant les bœufs
For all intents and purposes
Getting one's hands dirty: Mettre ses mains dans le cambouis
I am all astonishment
I am quite determined
I beg you would excuse me
I don’t want no <something>
I have an ace up my sleeve: J'ai un as dans ma manche
I reckon it will come back to me
I shall trespass on your time no longer
I take my leave
I'm fully aware
I'm much exceedingly obliged
If you can help it: Si tu peux
In broad dailight : En plein jour
In shambles : En pagaille
In your stead: À ta/votre place
It cannot be helped
It's a piece of cake: Facile
It's raining cats and dogs: Un temps de chien
Keenly: Vivement
Kill two birds with one stone: D'une pierre deux coups
Last I checked
Let the cat out of the bag: Vendre la mèche
Make haste!
Mind your own business
Much obliged: Infiniment reconnaissant
My own better judgment
Neither of us
No please, don't trouble yourself
Nohow: Aucunement
None better
On a whim: Sur un coup de tête
Please convey my regards to
Save your breath
Seldom: Jamais
Somewhat: Quelque peu
Still waters: Eaux calmes
Sweetheart, sweetie
The blue book: Le bottin mondain
The charges levelled against someone: Les charges portées contre quelqu'un
The task will fall on you
Thither: Là
To arise chiefly
To ask after someone: Demander des nouvelles de quelqu'un.e
To be ill qualified
To be in the know for
To be off-limit
To be on par with
To be out of one's wits
To be shacked up: Cohabiter
To be torn over something
To be up to speed with
To be witty: Être spirituel
To behove: Être bon
To break stride: Ralentir un élan
To cut to the chase
To feel blue
To flog a dead horse: S'acharner, Perdre son temps
To get a word in edgewise: Arriver à placer un mot
To give somebody the heave-ho:  To take someone's job away from them, usually because that person has done something wrong
To go astray: S'égarer
To go out of one's wits: Perdre la raison
To have a will of iron
To intrude upon your privacy
To keep down the fluff
To let off the hook: Laisser partir
To make something easy on someone
To sow the seeds of discord: Semer la discorde
To stand idly by
To take a rain check: Remettre une rencontre à plus tard
To take delight in something
To tear asunder: Déchirer en morceaux
To think ill of you
To wriggle out of something: To avoid doing something that you do not want to do
Unwillingly: Contre son gré
Utterly: Tout à fait
What about him?
What's on your plate?
Whilst: Tandis que
With all due respect
Would you be so kind as to
You did good
You got a mouth on you boy
Inner child
To weed out: Éliminer
Out of the blue: Inopinément, De but en blanc
To drive home the obvious
To get a word in edgewise: placer un mot
To sustain minor injuries
To put it bluntly
In the rough: À l'état brut
To be self serving
The bee's knees: la crème de la crème
To put people in harms way
To jump through hoops: To go through an elaborate or complicated procedure in order to achieve an objective
To go out on a limb: Prendre des risques
When push has come to shove
To leave someone to his own device
To give the latest (news)
To wash away one's tear
To be whole can of worms
In times past
To be long overdue
The chicken has come home to roast
To have absolutely no inkling : N'en avoir absolument aucune idée
To jump on to the wagon
To be dead in the water
Every cloud has a silver lining: À quelque chose malheur est bon
Believe it or not
On a hunch : Sur une intuition
To be just shy of
Unrequited love : Amour non-partagé
To continue in earnest : Continuer sérieusement
I kid you not : Je ne plaisante pas
I feel no further need to engage with you here.
At long last : enfin
in-kind support : aide en nature
something amiss : quelque chose qui cloche
A roller coaster : une montagne russe
A pet peeve : une bête noire
To be sorely lacking : manquer cruellement
Deriliction of duty : manquement au devoir
A sleight-of-hand : Un tour de passe-passe
Vast swaths : Vastes étendues
willy-nilly : bon gré mal gré
To bode well : Être de bon augure
To bode ill : Être de mauvais augure
To wind up : En finir avec, Liquider
A slam dunk : Un coup de poing
In excruciating details
To cut the mustard : Convenir, Faire l'affaire, Être à la hauteur
The jury is still out on whether or not ... 
Without further ado : Sans plus attendre
Second to none
To mix and match : mélanger et apparier, combiner
To come as no big surprise
A stroke of genius
To fell under one's sway : Tomber sous l'emprise de quelqu'un.e
Bless you : À tes souhaits
Out of spite : Par pure méchanceté
Nothing fancy : Rien d'extraordinaire
To follow suit : Suivre le mouvement
To bear the brunt : Porter le poids
To know the drill : Connaître l'exercice
To gloss over : Passer sous silence
To come to terms : Parvenir à un accord
To be tone-deaf : Ne pas avoir l'oreille musicale
To be long overdue : Être attendu depuis longtemps