You can picture someone whose arms end not in hands but in two thick hams. They can grab and shove, but they cannot pinch, fold, or hold something lightly. Everything they touch gets pressed too hard.
可以想成一個人手臂末端接的不是手，而是兩塊厚厚的火腿。他推得動、抓得住，卻捏不起小東西，也沒辦法輕輕放下什麼。凡是經過那雙手的東西，力氣都被加得太重。這個畫面同時解釋了兩件事：手上的笨，和做事的笨。
From the word’s history
Wiktionary records the word as ham (the thigh of a slaughtered animal, used as meat) plus fisted, probably picturing someone clumsy as if hams hung at the ends of their arms instead of hands.
來源記載這個字是 ham（動物後腿肉）加上 fisted（有…樣的拳頭），大概就是在形容一個人笨拙得像手臂末端掛著兩塊火腿。這條記載和今天的用法方向一致：問題不在力氣，在分寸。
Wiktionary, "ham-fisted"
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