A sitter can be thought of as the one that stays put while everything else moves. Someone holds a pose in a chair so a painter can work. Someone stays in the house with the children while the parents go out. A ball stops in front of an open goal and waits to be kicked. In each case the word points at whoever, or whatever, is staying in place.
sitter 可以想成「留在原位的那一個」。有人坐在椅子上不動，讓畫家慢慢畫。有人留在家裡陪小孩，等爸媽回來。球停在球門前面，等著被踢進去。周圍都在動的時候，這個字指的就是待在那裡不走的那一個。
From the word’s history
Wiktionary records sitter as Middle English sitter, sittere, that is sit plus -er: one who sits.
Wiktionary 記錄 sitter 來自中古英語的 sitter、sittere，等於 sit 加上 -er，也就是「坐著的那個人」。這條線索和上面的畫面接得上。
Wiktionary (kaikki extract, entry sitter/noun): "From Middle English sitter, sittere, syttare, equivalent to sit + -er."
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