Several parts combine into one character.

外 joins 夕 (evening) with 卜 (divination). In antiquity people read cracks burned into turtle shells (卜) to divine the future — normally in the morning. A divination cast in the evening (夕) was an exception that fell "outside" the proper schedule, and from that anomaly grew the meaning "outside, external" — the leading account.

Mandarin: wài, 4th tone. 外面 (wàimiàn, outside), 国外 (guówài, abroad), 外国人 (wàiguórén, foreigner), 外卖 (wàimài, food delivery — "sold outward"). It pairs with 内 (nèi, inside) — 内外 (nèiwài, inside and out).

Japanese: on-readings ガイ (gai) and ゲ (ge), kun そと (soto) and ほか (hoka). 外国 (gaikoku, foreign country), 海外 (kaigai, overseas), 外科 (geka, surgery — a rare ゲ reading), 外れる (hazureru, to miss / come off). This is the 外 (soto) of Japan's deep uchi/soto in-group–out-group distinction.

Korean hanja-sound "oe": 外國 (oeguk, foreign country), 海外 (haeoe, overseas), 野外 (yaoe, outdoors). It is the most basic spatial word, paired with 內 (nae, inside).

Memory aid: a divination (卜) cast in the evening (夕) — what falls outside the proper frame is "outside".