咬牙 literally means 'to bite one's teeth' and functions idiomatically as a verb meaning 'to grit one's teeth'—a physical expression of intense determination, suppressed anger, pain, or resolve in the face of difficulty. The character 咬 (yǎo) means 'to bite', and 牙 (yá) means 'tooth'; together, they evoke the involuntary clenching action people make when enduring hardship or steeling themselves for effort.

This phrase is commonly used in both spoken and written Chinese to describe mental or emotional fortitude under pressure—not just literal dental action. It often appears in contexts involving perseverance (e.g., studying late, pushing through fatigue), frustration (e.g., biting back anger), or stoic endurance (e.g., tolerating injustice). Unlike English 'grit one's teeth', 咬牙 rarely implies aggression; it emphasizes internal resolve more than outward hostility.