absum

Absum, abesse, afui, afuturus is a compound of sum and inherits every irregularity it has. Note the perfect: afui, not abfui, because b drops before f.

Three uses beyond the plain "be absent". Of distance it takes a measure in the accusative, oppidum milia passuum decem aberat. Of lacking it is close to desum. And tantum abest ut is a standing rhetorical formula, "so far is it from being the case that". The present subjunctive absit is a fixed expression of recoil - absit iniuria verbis, and the scholastic quod absit, "God forbid" - which is why you meet it far more often than the frequency of the verb would suggest. English absent and absence come from the participle absens.