
bitter
One of the basic tastes, perceived as sharp and often unpleasant, as with chicory, grapefruits, or black coffee.
Diese Schokolade ist mir zu bitter, ich mag lieber Vollmilch.
This chocolate is too bitter for me; I prefer milk chocolate.
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One of the basic tastes, perceived as sharp and often unpleasant, as with chicory, grapefruits, or black coffee.
Diese Schokolade ist mir zu bitter, ich mag lieber Vollmilch.
This chocolate is too bitter for me; I prefer milk chocolate.

Describes an experience, feeling, or realization that is painful, disappointing, or full of resentment.
Es war eine bittere Erfahrung, von seinem besten Freund betrogen zu werden.
It was a bitter experience to be betrayed by his best friend.

Describes very strong, penetrating cold.
Trotz des Sonnenscheins wehte ein bitterer Wind.
Despite the sunshine, a bitter wind was blowing.

deadly serious
Used as an intensifier to express that something is very serious, bad, or urgent. Often in the phrase 'bitterer Ernst'.
Das ist kein Scherz, es ist mein bitterer Ernst.
This is not a joke; I am deadly serious.

to badly need
A fixed expression meaning that something is needed very urgently.
Ich habe eine Pause bitter nötig.
I badly need a break.
Besides taste, 'bitter' very often describes painful feelings or disappointing events. Typical combinations are 'eine bittere Enttäuschung' (a bitter disappointment), 'eine bittere Niederlage' (a bitter defeat), or 'die bittere Wahrheit' (the bitter truth).
Es war eine bittere Erkenntnis, dass er den Job nicht bekommen würde.
It was a bitter realization that he would not get the job.
In connection with cold, 'bitter' acts as an intensifier and means 'very' or 'extremely'. People speak of 'bitterer Kälte' (bitter cold) or a 'bitterkalten' (bitterly cold) day.
Draußen herrschte eine bittere Kälte.
There was bitter cold outside.
The phrase 'Das ist mein bitterer Ernst' is used to emphasize that you mean something absolutely seriously and that it is not a joke.
Ich kündige meinen Job. Das ist mein bitterer Ernst.
I'm quitting my job. I am deadly serious.
The phrase 'etwas bitter nötig haben' means that you need something very urgently. Here, the word 'bitter' intensifies the urgency.
Nach dieser anstrengenden Woche habe ich ein Wochenende Ruhe bitter nötig.
After this exhausting week, I badly need a quiet weekend.
In the literal sense, 'bitter' is often connected with certain foods and drinks. These include, for example, 'bittere Schokolade' (bitter chocolate), 'bitterer Kaffee' (bitter coffee), or 'bittere Medizin' (bitter medicine).
Manche Salate wie Radicchio haben einen bitteren Geschmack.
Some salads, such as radicchio, have a bitter taste.
tart / harsh
Bitter describes a basic taste that is often pronounced and unpleasant. Herb describes a nonsweet taste nuance that is slightly bitter, sour, or astringent. bitter is appropriate for a clearly bitter taste, while herb is more suitable for a drier, subtler combination of these impressions.

Die dunkle Schokolade schmeckt deutlich bitter.
The dark chocolate tastes distinctly bitter.
Die dunkle Schokolade schmeckt herb und nur wenig süß.
The dark chocolate tastes tart and only slightly sweet.