Several parts combine into one character.

舅 is a phonetic-semantic compound: 臼 (mortar — providing the sound jiù) + 男 (man / male). The composite reads as "a senior male in the maternal line" — and the precise referent shifts dramatically across CJK languages. Korean and Chinese: maternal uncle (mother's brother). Japanese: father-in-law (husband's father). The same character names different relatives depending on which kinship grid you're reading.

Korean reading "gu." Korean uses 舅 for husband's father in classical compounds: 外舅 (oegu, husband's father — formal/archaic; modern Korean overwhelmingly uses 시아버지 sia-beoji), 舅父 (gubu, mother's brother — also archaic/formal). Modern Korean uses pure-hangeul (oesamchon, "outer-uncle" = mother's brother) for everyday usage; 구 vocabulary survives only in classical contexts.

Mandarin jiù, 4th tone. 舅 in Mandarin specifies the maternal uncle precisely: 舅舅 (jiùjiu, "uncle-uncle" = mother's brother — affectionate doubled form), 舅妈 (jiùmā, "uncle-mother" = mother's brother's wife / maternal uncle's wife), 舅父 (jiùfù, formal maternal uncle). Mandarin also uses 舅 in in-law contexts: 大舅子 (dàjiùzi, "big maternal-uncle" = wife's elder brother), 小舅子 (xiǎojiùzi, wife's younger brother). Mandarin's kinship grid distinguishes paternal uncles (伯/叔) from maternal uncles (舅) systematically.

Japanese on-reading キュウ (kyū) is rare. The kun-reading しゅうと (shūto) is what matters: 舅 (shūto, husband's father / father-in-law). Japanese pairs 舅 (shūto, FIL) with 姑 (shūtome, MIL) as a complementary kinship pair. The compound 嫁舅 (yome-shūto) describes the daughter-in-law and father-in-law relationship — though less stereotypically fraught than 嫁姑 (yome-shūtome, DIL-MIL), the FIL-DIL dynamic appears in Japanese fiction and family discourse.

Memory aid: 男 (man) plus 臼 (mortar — for sound) — a senior male relative on the kinship grid. Korean / Chinese: maternal uncle. Japanese: father-in-law.