One of the mysteries of the English language finally explained.
Something which is of principal importance in life; a livelihood; (now) especially the opportunity of enriching or otherwise benefiting oneself. Often in "to have an eye to (also for) the main chance": to have consideration for one's own interests; also "to be careful of the main chance".
Late 16th century (in an earlier sense). From main + chance, perhaps sometimes influenced by main.
main chance/ˈmeɪn ˌtʃans//ˈmeɪn ˌtʃɑːns/
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