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.
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 etymology supplied in this packet (kaikki:3f6f69a190036ae4:1122959046) for 'truant': Middle English truant/truand < Old French truant 'a beggar, vagabond; a rogue', probably Celtic
From the picture
溜掉這個動作
前一組看的是那個人，這一組看的是他怎麼離開座位的。同一幅畫面換個角度：不是「誰不在」，而是「他翹掉了」。英文有兩種說法走同一條路——一個是把 truant 包進固定片語 play truant，一個是直接把它當動詞用。兩者意思一樣，差別在語氣和場合。
noun · B2
To stay away from school without permission. This is the usual British English way of saying it, and the source marks it as British.
翹課；逃學
noun · C1
A student who stays away from school without permission — not ill, not excused, simply not there; the word is also used more widely for anyone who slips away from work or a duty they are supposed to be doing.
逃學的學生；翹掉該做的事的人