the.com/side bet

means A smaller, often informal wager made alongside a main bet, usually between individuals just for extra stakes or fun.

from Straightforward compound of 'side' (secondary, additional, off to the margin of the main event) plus 'bet.' It grew out of wagering culture, especially around gambling and sports, where people place a primary bet but can't resist adding a smaller, looser one on some tangential detail — who scores first, whether it rains, which horse places second. The phrase itself isn't ancient or exotic; it's just English doing what it does, bolting 'side' onto a noun to mark it as the lesser, unofficial cousin of the main action.