The main German word for “weak”, in the sense of lacking physical strength or energy, is “schwach”. Its opposite, “strong”, is “stark” in German. The corresponding nouns “weakness” and “strength” are “Schwäche” (f.) and “Stärke” (f.), respectively:

– “Ich fühle mich so schwach. Ich muss etwas essen.” – “I feel so weak. I have to eat something.”
– “Der Typ ist echt stark.” – “That guy is really strong.”
– “Ich habe eine Schwäche für Schokolade.” – “I have a weakness for chocolate.”
– “Ihre Stärke ist Mathe.” – “Math(s) is her strength.”
– “Der Geist ist willig aber das Fleisch ist schwach.” – “The spirit is willing but the flesh is weak.”

Note that “strong” and “strength” can also be translated as “kräftig” and “Kraft” in the sense of “powerful” and “power” (or “force”), respectively:

– “Das ist ein kräftiges Argument.” – “That’s a strong/powerful argument.”
– “Er ist die treibende Kraft im Büro.” – “He’s the driving force in the office.”

Relatedly, “weak point” is “Schwachstelle” (f.) and “weak willed” is “willensschwach”. Lastly, note that the noun “Stärke” can also refer to “starch”.