Definitions:

1. A person who bites their fingernails, often due to nervousness, anxiety, or habit. NOUN
   • She's been a nail-biter since childhood whenever she's stressed.
2. A tense, suspenful, or very close situation, especially in sports, competitions, or decision-making. NOUN
   • The election results were a nail-biter right to the end.

Synonyms:

• cliffhanger
• close call
• heart-stopper