Ilicet is a Latin word that would still have been recognized by many literate English speakers in the 1800s, but has since faded into obscurity. While it is idiomatic in usage, it comes from the Latin words īre, meaning 'to go' and licet, meaning 'it is permitted'; a literal translation would be something like 'it may go', although in application the tone was more along the lines of 'I would prefer it if it went thus'.
However, over time ilicet dropped its use as a verb and came to be used primarily as an adverb. In this case it usually meant 'all at once', 'suddenly', 'immediately', or 'straightaway', although at times it simply meant 'it is done'. It is in these various senses that it is most often seen in Latin. It was perhaps best known, at least among British schoolboys, in the phrase ilicet malam crucem, which is very loosely translated as 'to hell with you' (lit. 'off with you to the bad cross').
This word also occasionally appears in poetry as a reference saying goodbye to the dead. In Roman funeral ceremonies the praefica (head mourner) would formally dismiss the mourners with the cry of ilicet, and the mourners would say goodbye to the deceased with vale (goodbye, lit. 'be well'). This was the last goodbye to the dead, and the end of the funeral ceremony.