無念
むねん · munen · JLPT N1 · noun
regret
無 = nothing + 念 = thought or feeling: a regret with no thought left that could fix it
The same 念 appears in 残念 (zannen, a pity or disappointment). 無念 is the stronger form: a bitter regret where nothing more can be done.
念 — 今 (now, phonetic) + 心 (heart)
念 is a semantic-phonetic compound: 心 (heart) gives the meaning, while 今 (now) supplies the sound. The original sense was 'what is on the heart right now' — a thought held in mind. From this came the modern meanings 'to think of,' 'to miss,' and 'to recite' (holding words in mind).