schlapp
weak
To feel physically weak, tired, and without energy, often as a result of illness, exertion, or heat.
Nach der Grippe fühlte ich mich noch eine ganze Woche lang schlapp.
After the flu, I still felt weak for a whole week.

weak / limp
Loading your saved state.
weak
To feel physically weak, tired, and without energy, often as a result of illness, exertion, or heat.
Nach der Grippe fühlte ich mich noch eine ganze Woche lang schlapp.
After the flu, I still felt weak for a whole week.

'Schlapp' (weak/limp) is very often used as a predicate adjective with verbs like 'sein' (to be) or 'sich fühlen' (to feel). The construction 'Ich fühle mich schlapp' (I feel weak) is especially typical.
Nach dem langen Arbeitstag war ich einfach nur schlapp.
After the long workday, I was simply exhausted and weak.
The word 'schlapp' (weak/limp) belongs more to colloquial language. It sounds natural in conversation, but in formal or written contexts, other words such as 'schwach' (weak) or 'kraftlos' (powerless) would often be used.
Mir ist heute so schlapp, ich glaube, ich werde krank.
I feel so weak today; I think I'm getting sick.
The meaning of 'schlapp' (weak/limp) depends on the subject. If it refers to a person, it means 'müde/schwach' (tired/weak). If it refers to an object (e.g. a plant), it means 'schlaff/hängend' (limp/hanging). If it refers to something abstract (e.g. a performance), it means 'unzureichend' (insufficient).
Der Salat ist in der Sonne ganz schlapp geworden.
The lettuce became completely limp in the sun.
When a person feels 'schlapp' (weak), the typical causes in the sentence are often illness, intense heat, or physical exertion.
Bei dieser Hitze fühlt man sich den ganzen Tag nur schlapp.
In this heat, you just feel weak all day.
firm / fixed
'fest' and 'schlapp' are opposites when something hangs tautly or limply. Here, 'fest' means that something is tight, strong, or under tension. 'schlapp' means that something is not taut and hangs down limply. This fits, for example, with a strap, a tire, or a plant. Important: 'schlapp' can also mean tired, but in this relation it is about the opposite of taut or tight.

Nach der Hitze hängen die Blätter der Pflanze schlapp herunter.
After the heat, the plant's leaves hang down limply.
Bitte zieh das Seil fest, damit die Plane nicht wegfliegt.
Please pull the rope tight so the tarp doesn't blow away.