Origins
The word 'even' is one of the oldest in English, traceable without interruption from Old English to the present, and its semantic history illustrates how a concrete physical adjective can generate an abstract grammatical function through a chain of metaphorical extensions.
Old English 'efen' (also spelled 'efn') meant 'flat,' 'level,' 'equal,' or 'like.' It descended from Proto-Germanic *ebnaz, meaning 'flat' or 'level,' a word preserved across the Germanic family: German 'eben' (flat, level, even; also a discourse particle meaning 'exactly' or 'just'), Dutch 'even' (even, equal; also a particle meaning 'briefly' or 'just'), Swedish 'jämn' (even, smooth), and Old Norse 'jafn' (equal, even). The further pre-Germanic etymology is uncertain; some scholars connect it to PIE *h₁emp- (to be flat), but the reconstruction is tentative.
The adjective developed several branches of meaning in Old English and Middle English. The physical sense of 'flat' or 'smooth' (an even surface) was primary. From this came 'equal' (an even match, an even split), which required no great metaphorical leap — a level surface is one where no point is higher than another, and equality is the abstract equivalent. From 'equal' came the mathematical sense: an even number is one divisible into two equal parts. The phrase 'get even' (to achieve equality in a contest or to take revenge) also flows from this equality sense.
Old English Period
The emphatic adverb — 'even the king,' 'even worse,' 'not even once' — represents the most abstract extension. Its logic rests on the concept of leveling or equalizing. When a speaker says 'even the king feared him,' the effect is to bring the king down to the same level as ordinary people with respect to fear — to erase a distinction that the listener would normally expect. The adverb marks something as surprising precisely because it equates things that should be unequal. This use was already present in Old English but became far more common in Middle and Early Modern English.
A common misconception is that 'evening' — the time of day — derives from 'even' in the sense of the day being evened out or balanced between light and dark. This is folk etymology. 'Evening' comes from Old English 'ǣfnung,' from 'ǣfen' (evening, the time before night), which is etymologically distinct from 'efen' (flat, equal). The two words had different vowels in Old English — 'ǣ' versus 'e' — and came from different Proto-Germanic roots. Their modern English spellings happen to look similar, but this is accidental convergence, not shared origin. Confusingly, 'eve' and 'even' (as in Christmas Eve) do come from 'ǣfen,' the evening-word, adding another layer of homonymy.
The compound 'even-handed' (impartial, fair) dates from the sixteenth century and transparently combines the 'equal' sense of 'even' with 'hand,' suggesting someone who gives with both hands equally. 'Even-tempered' follows the same logic — a temperament that remains level, without extremes. 'Uneven' simply negates the base adjective.