
fade
bland
Without flavor or with very little flavor; not seasoned enough.
Die Suppe schmeckt etwas fade, es fehlt Salz.
The soup tastes a bit bland; it needs salt.
bland / dull / weak
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bland
Without flavor or with very little flavor; not seasoned enough.
Die Suppe schmeckt etwas fade, es fehlt Salz.
The soup tastes a bit bland; it needs salt.

dull
Boring, uninteresting, and without appeal or variety.
Die Party war ziemlich fade, deshalb bin ich früh gegangen.
The party was pretty dull, so I left early.

weak
Used to describe an excuse, explanation, or pretext as weak, unbelievable, and not very convincing.
Das war nur eine fade Ausrede, um nicht mitkommen zu müssen.
That was just a weak excuse so he wouldn't have to come along.
The German adjective 'fade' means 'geschmacklos' (tasteless/bland) or 'langweilig' (boring/dull). It should not be confused with the English verb 'to fade' (verblassen, schwächer werden).
Ohne Gewürze schmeckt der Reis ziemlich fade.
Without spices, the rice tastes pretty bland.
The exact meaning of 'fade' depends on the noun it describes. With food, it means 'geschmacklos' (tasteless/bland); with events or people, 'langweilig' (boring/dull); and with excuses, 'unglaubwürdig' (unbelievable/unconvincing).
Die Suppe ist fade.
The soup is bland.
In relation to food, 'fade' is often used with 'schmecken' (to taste) or describes typical foods such as 'fade Suppe' (bland soup) or 'fades Essen' (bland food).
Mir schmeckt das Gemüse zu fade, ich brauche mehr Salz.
The vegetables taste too bland to me; I need more salt.
When something is described as boring, typical combinations are 'eine fade Party' (a dull party), 'ein fader Witz' (a dull joke), or 'ein fader Abend' (a dull evening).
Das Gespräch wurde schnell fade, weil er nur über seine Arbeit sprach.
The conversation quickly became dull because he only talked about his work.
The combination 'eine fade Ausrede' (a weak excuse) is a very fixed and common phrase. It expresses that an excuse or explanation is transparent and not convincing.
Er kam mit einer faden Ausrede, warum er zu spät war.
He came up with a weak excuse for why he was late.
weak
For an unconvincing excuse, both schwach and fade are possible. Schwach generally assesses how unconvincing it is, while fade emphasizes an especially flimsy and unbelievable excuse. Use schwach neutrally, but use fade as a fixed, clearly derogatory characterization.

Seine Ausrede war fade und unglaubwürdig, denn sie passte überhaupt nicht zu den Fakten.
His excuse was flimsy and unbelievable because it did not fit the facts at all.
Seine Ausrede war schwach, weil sie meine Frage nicht beantwortete.
His excuse was weak because it did not answer my question.