bettor (n.)

1590s, "pledge as a forfeit to another who makes a similar pledge in return," originally in the argot of petty criminals, a word of unknown origin; see bet (n.), which appeared about the same time.

The intransitive sense of "lay a wager" is from c. 1600. It has been used since mid-19c. in various American English slang assertions (bet your life, 1848; bet your boots, 1856; you bet "be assured," attested by 1857 and identified in Century Dictionary as "originally California slang").

late 12c., "that which is better," from better (adj.). The specific meaning "one's superior" is from early 14c. The better "improvement" (as in for the better) is from 1690s. To get the better of someone "obtain mastery or victory over" is from 1650s, from better in a sense of "superiority, mastery," which is recorded from mid-15c. Related: Betters.

"one who lays wagers;" see bettor.