He cast a furtive glance at his phone, afraid the teacher would notice.

furtive 是形容词，来自拉丁语 furtivus（偷窃的），词根 furt- 表示"偷窃"。furtive 形容偷偷摸摸、鬼鬼祟祟的行为或表情，常带有害怕被发现的心理。