early 15c. (implied in diagonally), "extending as a line from one angle to another not adjacent," from Old French diagonal, from Latin diagonalis, from diagonus "slanting line," from Greek diagonios "from angle to angle," from dia "across, through" (see dia-) + gonia "angle" (from PIE root *genu- (1) "knee; angle").
As a noun, from 1570s, "straight line drawn from one angle to or through another not adjacent, in a plane or solid figure." In chess, "a line of squares running diagonally across a board."
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