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.
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."
Two branches
被悄悄推過來的消息
這一組直接用了那個「輕輕一撥」的畫面：原本在暗處的事實，被推到某個人手上。差別只在推的是誰——第一個意思是人在推，第二個意思是情況自己漏了出來。
noun · B2
A piece of secret information, or a warning, that someone passes quietly to the police or to another authority, usually about something illegal that is happening or is about to happen.
密報／線報
球場上被撥出去的那一球
這一家把「輕輕一撥」從消息換成了真正的球。動作是一樣的：不是抱著走，是碰一下讓它到你這邊。這是概念上的連結，方便記住，不是在主張它從別的意思演變而來。
noun · B2
In basketball, the start of a game, when the referee throws the ball straight up between two players and they jump to tap it to their own team; it is also used for the moment the game begins.
跳球開賽