From
柳永's
《蝶恋花·伫倚危楼风细细》 (Northern Song ci):
衣带渐宽终不悔,
为伊消得人憔悴. Originally a love poem - the speaker grows thin from longing for a distant beloved.
王国维 in
《人间词话》 elevated it to the ‘second of three realms’ of scholarly achievement, giving it a completely new life as a phrase about devoted effort.