髭 is a phono-semantic compound: 髟 (the long-hair radical, indicating any hair-like growth) + 此 (cǐ, "this" — phonetic). 此 lent the sound, 髟 carried the meaning. Specifically: hair growing above the mouth = mustache. CJK distinguishes three beard zones with three different characters — 鬚 (chin beard), 髭 (mustache), 髯 (sideburns) — a level of facial-hair vocabulary granularity that English entirely lacks.

Korean reading "ja." Almost a dead character in modern Korean — the Hangul word (suyeom) covers all beard zones, and 髭 appears only in dictionaries and classical texts. A Korean asked to read 髭 will often pause before producing the sound.

Mandarin zī, 1st tone. 髭 itself appears almost exclusively in literary Chinese; everyday Mandarin uses 胡子 (húzi, "the beard / mustache" — generic) for the general concept and 小胡子 (xiǎohúzi, "small beard" = mustache) for the specific. The 子 suffix turns the body-part into a casual everyday noun — a grammatical move Mandarin loves.

Japanese on-reading シ (shi) — almost never used. Kun-reading ひげ (hige) — 髭 (hige, mustache), 髭剃り (higesori, shaving). Modern Japanese sometimes writes all three beard-zone characters (鬚, 髭, 髯) with the same kun reading ひげ, distinguishing them only through which character is written — when a Japanese person says "hige" out loud, only context tells you which beard zone.

Memory aid: hair (髟) plus "this" (此, phonetic) — "this hair, right here above the mouth."