You can think of truant as an empty seat with somebody wandering somewhere else. The seat was expected to be filled, nobody agreed to the absence, and the person is not ill — they are simply off, somewhere the day was not meant to be spent. Everything the word does comes out of that one picture: the person missing from the seat, the act of slipping away from it, and the drifting quality of anything that will not stay where it belongs.
From the word’s history
Through Middle English truant from Old French truant 'a beggar, vagabond; a rogue', probably of Celtic origin. The old word named someone with no fixed place at all.
經由中古英文 truant，來自古法文 truant，意思是乞丐、流浪者、無賴，來源可能是凱爾特語。老字講的是一個整輩子都沒有固定落腳處的人；後來範圍縮小到一個上午——該在教室裡，人卻在別處晃。這一段是資料裡明確記載的字源，不是從教學上的比喻推回去的。
Wiktionary, "truant"
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