knock/throw you for a loop

リーダーズダイジェスト5月号の "BOUNCE BACK CHRONICLES" と題した記事にあった表現です。
Sometimes life throws you a curveball, hands you a lemon, or knocks you for a loop."knocks you for a loop" の意味が分かりませんが、何か慣用句の様な気がします。
最初に "knock" で辞書を引きました。
・Cambridge Idiom Dictionary: if something that happens knocks you for a loop, it upsets or confuses you because you do not expect it He knocked me for a loop when he said he was quitting his job.
・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs:
1. Fig. to strike someone hard. You really knocked me for a loop. I hope that was an accident. DI was really knocked for a loop by the falling branch.
2. and throw someone for a loop Fig. to confuse or shock someone. (This is more severe and upsetting than throw someone a curve.) When Bill heard the news, it threw him for a loop. The manager knocked Bob for a loop by firing him on the spot.
・American Herritage Dictionary: Slang To surprise tremendously; astonish.
次ぎの辞書は "loop" の項に次ぎの説明がありました。
Merriam-Webster: for a loop : into a state of amazement, confusion, or distress threw us for a loop
Yes, sometimes life knocks me for a loop.
"knock/throw you for a loop" の慣用句として覚えましょう