可
か · ka · JLPT N3 · noun
passable
可 (ka) is the lowest passing grade; add 不 and 不可 (fuka) means fail
Japanese universities grade 優, 良, 可, then 不可. 可 means just good enough to pass, and 不 (not) in front turns it into a fail. The same 可 shows up in 可能 (kanō, possible): if something is 可, it is allowed or it can be done.
可 — mouth 口 + breath marker 丁 (modified)
可 originally depicted a mouth (口) with a curved stroke suggesting exhaled breath or speech, conveying approval or assent. From this came the meaning 'to permit' or 'can,' as granting permission was expressed verbally. Today it broadly means 'may,' 'can,' or 'approve.'