A helpful picture is a person standing with no strings on them. Something moves them from the inside — they want it — and nothing outside is pulling. That is what the two words say when you read them apart: will is the wanting, and free is the no-strings part. Once you can see the strings, both uses fall out of the same picture: asking whether anyone tied this particular choice today, and asking the larger question of whether there are strings on us at all.
From the word’s history
Wiktionary records free will in Middle English as fre will, fre wil, fre wille — the same two words, already joined. Read literally that is 'unbound wanting', which is exactly the no-strings picture both modern uses still rest on.
Wiktionary 記到中古英文就已經寫成 fre will、fre wil、fre wille，兩個字那時候就在一起了。字面直翻就是「沒有被綁住的意願」，剛好就是今天兩個用法都還靠著的那張「沒有線」的圖。
Wiktionary entry for 'free will', etymology_text supplied in this packet: 'From Middle English fre will, fre wil, fre wille.'