One way to see it: somebody was measuring out how much to give, and held a bit back. What arrives is real but short of what it should be. Food, notes, evidence, cloth, it is always the same act of holding back, and the word carries a small complaint about it.
可以想成有人在量「要給多少」，然後手一收，留了一點沒給出去。送到你面前的東西是真的，但明顯比該有的少。飯、筆記、證據、布料，都是同一個「留一手」的動作，而這個字本身就帶著一點對這件事的抱怨。
From the word’s history
The source gives the formation directly: skimp plus -y. Skimp is to save by giving less than you should, and -y turns that act into the look of the thing it produces.
來源直接說明了組成：skimp 加 -y。skimp 是「該給的省下來」，加上 -y 就變成形容那個動作做出來的結果。
Wiktionary etymology supplied in this packet (kaikki:3f6f69a190036ae4:1136905388:13215)
登入就可以看每一支怎麼連回核心畫面。