neck and neck At the same level or position as someone else in a race or competition; neither side is ahead.
neither here nor there Not relevant or important to the current situation; doesn’t affect the main point.
never a dull moment There is always something happening; life or a situation is constantly interesting or exciting.
new kid on the block A person or thing that is new to a place, group, or situation and not yet familiar or established there.
nip it in the bud To stop a problem or bad situation at an early stage before it grows or becomes harder to control.
no hard feelings There is no resentment or ill will toward someone after a disagreement or hurt; don't be upset about it.
no strings attached With no conditions, obligations, or hidden requirements; freely given or offered.
no use crying over spilled milk Don’t waste time being upset about something that already happened and can’t be changed; focus on what to do next.
not my cup of tea Something you don’t like, don’t enjoy, or aren’t interested in; not suited to your taste or preference.
not playing with a full deck To be unintelligent, irrational, or mentally unwell; lacking good judgment.
not the sharpest tool in the shed Not very intelligent or quick to understand; somewhat dull or foolish.
nothing to sneeze at Not trivial or unimpressive; something that deserves respect or serious consideration.
off the beaten path In a place that is not often visited or is far from the usual routes; unusual or unconventional compared with what most people do.
off the beaten track In a place that is not often visited; away from the usual routes, mainstream, or common choices.
off the hook No longer responsible, blamed, or in trouble; released from an obligation or consequence.
off the record Not for publication or official reporting; said in confidence and not to be attributed.
off the top of my head From memory and without careful thought or checking facts; a spontaneous quick answer.
off the top of your head From memory and without careful thought or checking; an immediate, unprepared answer.
on a roll Having a streak of success or good luck; performing very well repeatedly without interruption.
on thin ice In a risky or precarious situation where one mistake could lead to serious trouble or failure.
once and for all Finally and definitively, so the matter is settled permanently and won’t need to be dealt with again.
one of these days At some time in the near or not-too-distant future (often vague; sometimes used as a mild warning).
open a can of worms To start dealing with a complex issue that creates many new problems or complications.
open pandora’s box To do something that triggers many unexpected problems or complications that are hard to control.
open the floodgates To allow something to start happening in large amounts, often suddenly and hard to control.