The word 竟然 is formed through the classical Chinese method of compounding, where two independent characters are combined to create a new, nuanced meaning. The first character, 竟, originally meaning "to finish" or "to complete", carries an evolved sense of something reaching its final end or conclusion. The second character, 然, functions as a suffix meaning "so" or "thus", often used to create adverbs. When fused, the literal sense is "having reached such a (final) point", which conceptually evolved to express that a situation has actually and unexpectedly arrived at a particular outcome, hence conveying surprise or disbelief at the realized fact.