頓
トン, トツ, にわか.に, とん.と, つまず.く, とみ.に, ぬかずく

suddenly, immediately, in a hurry

JLPT0 G7 S13 F5000

As per 屯# (swollen) + 頁 (group of people) → camp swollen with soldiers (compare 軍 and 營) → *stop*; *settle* (down) (← form a camp) → *arrange*. *Prostrate oneself*, *bow deeply*, *suddenly*, *hastily* and *all at once* are borrowed meanings.

[Full Kanji Etymology List]