handhold / firm grip / bond
Feminine noun. الْعُرْوَةُ الْوُثْقَى (the firmest handhold) is a famous Quranic metaphor. From عَرَا (to occur/attach). وُثْقَى is feminine of أَوْثَقُ (firmest).
This verse contains the famous phrase 'lā ikrāha fī ad-dīn' (no compulsion in religion) - faith must be a free choice, and that choice is the firmest handhold.