Think of the word 'tip' as a light touch — one finger, one quick tap, just enough to move something. A tip-off is that tap. Something that was sitting where you could not reach it gets nudged across to your side: a fact you were not meant to know, a detail the situation lets slip, or a ball in the air that one hand sends to your team. Nothing is carried over to you; it is only touched, and then it is yours.
From the word’s history
Our source records tip-off as a noun formed from the two-word verb 'tip off' — packaging the action of quietly telling someone into a single thing you can receive.
來源記載 tip-off 是從動詞片語 tip off 變來的名詞（deverbal）。tip off 是「私下透露給某人」，把這個動作整個包成名詞，就成了「那則被送過來的消息」。來源只說到這一層，再往前的字源這裡沒有資料。
Wiktionary entry 'tip-off' (packet source_ref kaikki:3f6f69a190036ae4:966888400:6248), etymology_text: "Deverbal from tip off."
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