Character 甥

甥 meaning in Japanese is nephew: a sibling's son.

甥 meaning in Chinese is narrower: 甥 and 外甥 point to a sister's son, while a brother's son is 侄. So Japanese 甥 covers both sides; Chinese splits them.

甥 origin is as a Chinese kinship word taken into Japanese. Japanese dictionaries cite it from an 8th-century text, with a later example in The Tale of Genji in the early 11th century.

Why can a dictionary give another meaning?

Older use also meant son-in-law, but nephew is the everyday sense.

In Japanese, 甥っ子 meaning is simply nephew said warmly: use 甥っ子 for family talk and bare 甥 for forms and records. The pair is 甥 for nephew and 姪 for niece.