In old documents, one can find the expression 'I have understood everything.'
Literary adverb meaning 'all' or 'every one'. 悉皆 is even rarer and more archaic.
complete enumeration; full-count survey; census
The historical derivation is uncertain; the kanji 悉 (all, exhaust) and 皆 (all, everyone) combine to reinforce the meaning 'all'.