deppa 出っ歯 (Buckteeth)

In Japanese, deppa 出っ歯 means having upper teeth that protrude forward, ending up outside the mouth.

Literally, deppa 出っ歯 means "leaving teeth." A "tooth" or "teeth, ha 歯, that "leaves," deru 出る, the mouth. The ha は becomes ppa っぱ due to handakuonka 半濁音化.

Ishino Arashi 石野あらし, example of buckteeth, deppa 出っ歯.

Character: Ishino Arashi 石野あらし
Anime: Game Center Arashi, ゲームセンターあらし (Episode 1)

Translation

A translation for deppa in English would be "buckteeth" or "overbite," but there's a few things worth noting.

First, deppa means specifically the upper teeth leaving mouth.^{[出っ歯 - kotobank.jp, accessed 2020-04-28]}

Non-technically, "overbite" is when the upper teeth are forward, "underbite" is when the lower teeth is forward, and "buckteeth" can mean either upper or lower protruding teeth.

Technically, however, in dentistry "overbite" is how much the upper teeth cover the lower teeth vertically, while "overjet" is the distance between the upper and lower teeth.

Also note that deppa is a type of malocclusion (bite misalignment). In anime, a similar, but far more common type of malocclusion would be the yaeba 八重歯, which typically refers to a protruding canine, also known as the anime fang.