if the shoe fits If a description (often negative) matches you, you should accept it as applying to you.
in black and white In written or printed form, clearly stated (often as proof or an official record).
in broad daylight In the daytime, in full light and public view—often implying boldness or lack of secrecy (e.g., a crime done openly).
in cahoots Working together closely, often secretly and usually for something dishonest or questionable.
in full swing Happening at its busiest or most active stage; operating at maximum speed or intensity.
in hot water In trouble or facing criticism, punishment, or serious difficulty because of something you did or failed to do.
in the bag Certain to be achieved; already secured or guaranteed (often about a win, deal, or result).
in the doghouse In trouble or out of favor with someone (often a partner) because you did something wrong.
in the heat of the moment While emotionally excited or upset, acting without time to think carefully.
in the limelight Receiving a lot of public attention; being the focus of interest, often due to fame or publicity.
in the long run Over a long period of time; eventually, considering the final outcome after time has passed.
in the nick of time Just in time; at the last possible moment before something happens or it becomes too late.
in the right place at the right time Being somewhere at the optimal moment so you benefit from luck or opportunity.
in the same boat In the same situation as someone else, usually facing the same difficulties or limitations.
in the thick of it Deeply involved in the busiest, most intense part of an activity, situation, or trouble.
in your element Feeling comfortable, confident, and performing your best in a situation that suits you.
iron out To remove problems, disagreements, or small faults by working through them and making things smooth.
iron out the wrinkles To resolve small problems or difficulties and make a plan, process, or arrangement work smoothly.
it ain't over till it's over You shouldn’t assume the outcome yet; the situation isn’t finished and can still change.
it takes two to tango A situation, conflict, or outcome usually involves two parties; both share responsibility or participation, not just one side.
ivory tower A state of being isolated from practical reality, especially in academia or intellectual circles.
jack of all trades A person who can do many different kinds of tasks, often without being an expert in any one of them.
jump down someone's throat To react angrily and suddenly to someone, criticizing or yelling at them harshly (often over something small).
jump on the bandwagon To join a popular trend, movement, or opinion because it’s gaining support, often without much original commitment.
jump the gun To do something too soon or start before the appropriate time, often causing problems or confusion.
jump through hoops To go through many difficult, often unnecessary steps or bureaucratic requirements to achieve something.
keep a straight face To manage not to laugh, smile, or show emotion, especially in a funny or awkward situation.
keep an eye on To watch someone or something carefully for a period of time, to prevent problems or notice changes.
keep at bay To prevent someone/something from approaching or having an effect; to hold off or control a threat/problem.
keep the wolf from the door To avoid hunger or severe financial hardship; to have just enough money to survive.
keep up appearances To maintain an outward image of success or normality, often hiding problems or true feelings.
keep your chin up Stay optimistic and not get discouraged, especially during difficulty or disappointment.
keep your fingers crossed To hope for good luck or a favorable outcome, often while waiting for results, sometimes accompanied by the gesture of crossing fingers.
keep your head above water To manage to survive financially or emotionally, avoiding failure despite difficulties; to cope just enough to get by.