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.
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.
