勝手に
かってに · katteni · JLPT N2 · noun
arbitrarily
🎲 勝手 = winning hand; 勝手に = doing it on your own whim, as you please, arbitrarily
勝手 (katte) means 'one's own convenience or will'. Add に and it becomes an adverb: acting by your own will, with no fixed rule. Think of an unplanned dice roll.
手 — pictograph of a hand with five fingers
手 is a pictograph showing an open hand with fingers and a wrist. Early bronze and oracle bone forms clearly depict five splayed fingers, which simplified over time into the modern shape. It keeps its original meaning of 'hand' and also serves as a radical in many characters related to hand actions.