Douglas Harper's Etymology Dictionary
absit omen
Latin, literally "may this omen be absent."
Wiktionary
absit omen
interj. (context rare English) May what is said not come true. (First attested in the late 16th century.)(R:SOED5: page=9)
Usage examples of "absit omen".
GLOSSARY ABSIT OMEN Proverbial Latin saying: may there be no ill omens.
Months of calm before you - I touch upon wood: absit omen - months of mental calm, with only your few Leopards to look after, since for ourselves we have an excellent surgeon in young Mr McLean.