The term 'once upon a time' is often used to refer to a time in the past, usually in stories or fairy tales. The crossword solutions for this term include ERST (4 letters), ERSTWHILE (9 letters), AGO (3 letters), AGESAGO (7 letters), LONGAGO (7 letters), and YEARSAGO (8 letters). ERST is an archaic word meaning 'FORMERLY' or 'at first'. ERSTWHILE is an adverb meaning 'FORMERLY' or 'in the past'. AGO is an adverb meaning 'in the past'. AGESAGO is an adverb meaning 'long AGO'. LONGAGO is an adverb meaning 'long AGO'. YEARSAGO is an adverb meaning 'a year AGO'. All of these words can be used to refer to a time in the past, which is what 'once upon a time' is often used to mean.