The term 'butterfly, once' can be represented by two three-letter crossword solutions: TRA and PUA. TRA stands for the Latin word 'trans', meaning 'ACROSS' or 'BEYOND'. PUA stands for the Latin word 'puer', meaning 'BOY'. Together, these two words form the phrase 'trans puer', which translates to 'butterfly, once'. This phrase is used to describe a person who has gone through a transformation, such as a caterpillar turning into a butterfly.