"week" - This refers to a period of seven days. It could be considered a near-homonym if you were talking about feeling weak *all week*.
"wick" - This refers to the string in a candle. It's a much less common word and unlikely to cause confusion.
"whack" - This is a slang term for "hit" or "strike" and has no connection to the meaning of "weak".
Ultimately, there is no true homonym for "weak" because there is no other word that sounds exactly the same and has the same meaning.