Picture something that was holding its place and then lets go — snow sitting on a roof until it slides, a belt a wheel until it starts to turn without biting. The thing does not fall away completely. It moves off the spot it was supposed to keep, and you can measure how far. Every use of this word is that same small loss of grip, applied to something else: to a date, to a level, to a price.
From the word’s history
The word is slip plus the ending -age, which turns an action into a noun for the act or the amount of it — so slippage is literally 'the slipping', or 'how much slipping there was'.
這個字是 slip（滑）加上字尾 -age 組成的。-age 把一個動作變成名詞，常帶「這件事的發生」或「發生了多少」的味道。所以 slippage 字面上就是「滑掉這件事」，或是「滑掉了多少」。這正好說明它為什麼既能指事情發生，也能指幅度。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:1228702528:9320), entry slippage: "From slip + -age."
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