Green’s Dictionary of Slang

Monday morning quarterback n.

also (Saturday night and) Sunday morning quarterback
[amateur criticism of the week’s pro football matches, played on Sundays]

(US) a person who criticizes with the benefit of hindsight.

Arcadia (CA) Trib. 30 Oct. 5/4: But he found a Roman army there before him, with another close on his trail. He was licked, and he knew it. He didn’t spend any time in vain regrets. Spartacus was never a Sunday morning quarterback.
[US]N.Y. Times 5 Dec. 22: Barry Wood, Harvard’s all-America quarterback, mounted the ramparts in the role of a defender today [...] The answer to overemphasis was to be found not on the field, but in the stands, where sit what Wood called ‘the Monday morning quarterbacks.’.
[US]Sheboygan (WI) Press 17 Nov. 4/3: Are you one of those Saturday night and Sunday morning quarterbacks who knows what they ‘should have’ done to win the game?
[US]R. Chandler Playback 101: Common sense is the Monday morning quarterback who could have won the ball game if he had been on the team.
[UK]P. Theroux Picture Palace 32: I know all old people are Monday morning quarterbacks.