Where the shape comes from
A knife with a sharp, angled blade, once used to cut or brand the skin of criminals and slaves — the pain of the blade giving the word its sense of bitterness and hardship, and by taste, anything sharp, pungent, hot. It gives 辛苦, hardship, toil, literally bitter and laborious, and 辛辣, pungent, spicy, a flavour with the same edge as the old blade.
汉字 filed under 辛
No published character on HSK Guide is filed under this radical yet. 汉字库.