also: しずごころなし
ku-adjective (archaic)
Archaic 'ku' adjective describing a state of mental unrest or inability to stay calm. Not used in modern Japanese; found in classical literature.
古典では「静心無し」と書いて心が落ち着かない様子を表した。
In classical texts, '静心無し' was written to express a state of unease or restlessness.
Compound of 静 (shizu, 'quiet') + 心 (kokoro, 'heart/mind') + 無し (nashi, 'without'). The exact historical derivation is uncertain, but the word literally means 'without a quiet heart'.