(pathology) collagenosis, collagen disease

meeting all three important requirements

(fiction) Hanahaki, an illness affecting those consumed by unrequited love that manifests as the coughing up of flowers, which clears up upon reciprocation. A fictional illness, it has found popularity among both Japanese and foreign online fanfic (especially) artists and authors for its poetic quality.

flat / level / even / smooth

箕作柄長提灯鮟鱇: a triplewart seadevil

精神分裂病:

mysterious / strange, bizarre

(dated, potentially offensive) schizophrenia

unequal treaty (treaty forced upon weaker nations by imperialist power)

(literally) know the other side and yourself, a hundred battles will not be perilous