One way to hold this word is a coin spinning in the air. The moment it leaves the hand, nobody is deciding any more: the two faces have the same chance, and everyone just watches it land. Each use of toss-up keeps part of that moment — sometimes the real coin, sometimes only the feeling that two things are so equal that chance is all that is left.
From the word’s history
Wiktionary records toss-up as formed from the verb phrase toss up, and connects it to tossing a coin. So the noun starts at the coin itself, and the even-chance meaning follows from what a coin toss is like.
Wiktionary 記載 toss-up 來自動詞片語 toss up，並指出它跟丟銅板有關。所以這個名詞的起點就是那枚被丟上去的銅板；「兩邊機會一樣」的意思，是從丟銅板這件事本身長出來的。
Wiktionary (packet extract), entry 'toss-up' (noun): 'Deverbal from toss up, suggesting tossing a coin.' — kaikki:3f6f69a190036ae4:917406001:8591
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