When it comes to a "stubborn lock," there are several synonyms that come to mind. One synonym could be a "hardened lock," referring to the difficulty in opening the lock. Another option may be a "resistant lock," suggesting that the lock is putting up a fight against being unlocked. Alternatively, a "rigid lock" conveys the idea of a lock that is unyielding and inflexible. You could also describe it as a "stuck lock," indicating that the lock is stuck in place, making it difficult to move. Ultimately, the choice of synonym depends on the context of the situation and the particular characteristics of the lock in question.