
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
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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.

limp
Not taut or firm, but hanging down limply and without strength.
Die Blumen ließen bei der Hitze ihre Köpfe schlapp hängen.
In the heat, the flowers let their heads hang limply.

weak
Colloquial for a weak, insufficient, or unconvincing performance or excuse.
Das war eine ziemlich schlappe Vorstellung der Heimmannschaft.
That was a pretty weak performance by the home team.
'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.
weak
For a lack of physical energy, both schwach and schlapp are possible. Schlapp usually emphasizes a temporary feeling of tiredness and exhaustion, while schwach can also describe generally reduced physical strength. Use schlapp after exertion or illness and schwach for a general condition.

Nach dem langen Spaziergang war der Hund schlapp und legte sich sofort hin.
After the long walk, the dog was exhausted and lay down immediately.
Der alte Hund ist schwach und kann nicht mehr weit laufen.
The old dog is weak and can no longer walk very far.