1. An unconvincing or inadequate explanation or justification for one's actions or absence
His excuse for being late was weak and nobody believed him.
Sua desculpa por chegar atrasado era fraca e ninguém acreditou nele.
2. A reason that lacks credibility, substance, or legitimacy
That's a weak excuse for not doing your homework.
Essa é uma desculpa fraca para não fazer seu dever de casa.