Tricky."ER" is half of "peer", and "wag" is linked to "prank", but "wager" is the bet, not the better.
I wonder if it is "caper" for "prank", but I can't explain that either.
Like caper better, as explained by kilowatt and andyc.
I was justifying wager as a person who bets, hence 'better' but can see cap for better is better!