Not strong or lacking in power, ability, or stamina; can describe something faint (like weak tea/wine) or someone inexperienced or unskilled.
彼は体が弱いので、風邪をひきやすい。
He has a weak constitution, so he easily catches colds.
このワインはアルコールが弱く、飲みやすい。
This wine is low in alcohol and easy to drink.
彼は議論が弱いところがあるが、熱意はある。
His arguments are somewhat weak, but he has enthusiasm.
寒さに弱いので、冬は暖かい服を着る。
He is sensitive to cold, so he wears warm clothes in winter.