Onset

wiktionary

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From on- +‎ set. Compare Old English onsettan(“to impose; oppress, bear down”).

etymonline

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onset (n.)

1530s, "attack, assault, a rushing or setting upon," from on + set (n.); compare verbal phrase to set (something) on (someone), c. 1300, originally "sic (a dog) on." Weaker sense of "beginning, start" is recorded from 1560s. Figurative use in reference to a calamity, disease, etc. is from 1580s.