A simple picture holds this whole word: a mouth that opens wide and has nothing to bite with. Teeth are the part that grips, tears and hurts. Take them away and the mouth still works — it can open, it can make noise, it can smile — but it cannot close on anything. Both meanings of the word live inside that one image; only the mouth changes.
From the word’s history
Wiktionary traces the word to Old English tōþlēas, tooth plus -less, with the same pattern in Dutch tandeloos, German zahnlos and Danish tandløs. So the word has always said exactly what it looks like it says, which is why the picture is so easy to keep.
字典把這個字追到古英語 tōþlēas，也就是 tooth 加上 -less；荷蘭語 tandeloos、德語 zahnlos、丹麥語 tandløs 都是同一個拼法思路。所以它從一開始就只說了字面上的那件事：沒有牙。正因為這麼直白，那個畫面才這麼好記。
Wiktionary entry for toothless, supplied in this batch's evidence packet: From Middle English tothles, from Old English tōþlēas, equivalent to tooth + -less.
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