额外
éwài · HSK 7
extra
➕ 额 forehead + 外 outside = beyond the expected, i.e. extra
Think of it as going outside (外) what your forehead (额, also means quota/amount) planned for. When something exceeds the set amount, its extra — 额外.
外 — evening 夕 + divination 卜
Originally a compound of 夕 (evening) and 卜 (divination). Ancient Chinese divinations were performed in the morning; doing them at evening (夕) was considered irregular or 'outside' the norm. From this sense of deviation, 外 came to mean 'outside' or 'external'.