noun, suffix, adverb
exceedingly; extremely; very many
Used as an adverb or suffix to emphasize degree or quantity; often in set phrases like 千万無量 or 千万言を費やす. The 'indeed' gloss reflects an older emphatic use, now mostly literary.
彼の努力は千万無量だ。
His efforts are immeasurable.
その問題については千万言を費やしても足りない。
Even a myriad words would not suffice for that problem.
From the number 'ten million' (千万, せんまん), used figuratively to express a vast amount or degree. The reading せんばん is a euphonic change (rendaku) common in compounds.