Unhealthily thin is a term used to describe someone who is significantly underweight and has a body mass index (BMI) below the healthy range. This term can be described by three words: GAUNT (5 letters), BONY (4 letters), and SCRAWNY (7 letters). GAUNT means having a thin, haggard, or wasted appearance. BONY refers to having a body that is thin and angular with prominent bones. SCRAWNY means having a thin, weak, or emaciated appearance. All three words are used to describe someone who is unhealthily thin.