A helpful way to hold pert is: nothing tucked away. The word is a form of apert, which carried the idea of being open, so picture whatever the word describes as standing out instead of staying down. A face that shows all its energy is pert. A remark that says the quiet part out loud is pert, and that is where the cheekiness comes from. A nose or a pair of ears that lift instead of drooping is pert too. Same upward, uncovered picture; three different things doing the lifting.
From the word’s history
Wiktionary traces pert to Middle English pert, perte, an aphetic (clipped) form of apert. The packet also records an obsolete sense of pert meaning open, evident, unhidden, which lines up with that route and makes the 'nothing tucked away' picture easy to hold.
Wiktionary 記載 pert 來自中古英語 pert、perte，是 apert 被削掉開頭的形式。資料裡另外收了一個已經廢棄的意思：open、evident、unhidden（公開的、看得見的）。這剛好跟那條來源對得上，也讓「什麼都沒收著」這個畫面站得住。
Wiktionary, "pert"
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