noun
buckteeth; prominent front teeth; projecting teeth
Refers to front teeth that stick out noticeably. Often used in a descriptive or mildly teasing way, but not inherently rude.
彼は反っ歯がチャームポイントだ。
His buckteeth are his charming feature.
子供の頃、反っ歯を気にしていた。
When I was a child, I was self-conscious about my protruding teeth.
出っ歯 also means protruding teeth, but is more common and slightly more direct. 反っ歯 emphasizes the outward angle, while 出っ歯 simply means sticking out.
From 反る (そる, to bend backwards) + 歯 (は, tooth), with gemination and rendaku. The small-tsu spelling reflects the pronunciation.