flat
exhausted
Colloquial for completely exhausted, tired, and lacking energy.
Nach dem Marathon war ich komplett flat.
After the marathon, I was completely exhausted.

exhausted / flat
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exhausted
Colloquial for completely exhausted, tired, and lacking energy.
Nach dem Marathon war ich komplett flat.
After the marathon, I was completely exhausted.

The word 'flat' is borrowed directly from English. It is mainly used in colloquial speech or in specific fields such as design and technology.
Nach der langen Wanderung waren wir alle total flat.
After the long hike, we were all totally exhausted.
In the meaning 'erschöpft' (exhausted), 'flat' is almost only used predicatively, that is, with verbs like 'sein' (to be) or 'sich fühlen' (to feel). Using it directly before a noun (attributively) is unusual here.
Ich fühle mich heute einfach nur flat und brauche eine Pause.
Today I just feel exhausted and need a break.
In the context of design and user interfaces, 'flat' describes a minimalist style without 3D effects. Here it can also be used attributively, that is, before a noun.
Viele Apps nutzen heute ein sogenanntes „Flat Design“.
Many apps today use what is called 'flat design'.
just / exactly
'eben' and 'flat' can both describe something that is flat or has no elevation. 'eben' is the normal German word and fits many everyday situations, for example with a floor, a road, or a surface. 'flat' is an Anglicism and sounds more modern or technical. You hear it especially in design, technology, or style. Learners may confuse the words because both can mean 'flat'. In practice: for normal everyday things, people are more likely to say 'eben'; in design or technical texts, 'flat' may sound more natural.

Das neue Design der App ist sehr flat und benutzt kaum Schatten.
The app's new design is very flat and uses hardly any shadows.
Der Boden im neuen Zimmer ist eben, deshalb steht der Schrank stabil.
The floor in the new room is level, so the cabinet stands securely.