Prof Ibrox explains:

"Accidence is not a typo; it's a real word. Obviously, the joke is that accidence sounds like accidents. Accidence is the part of grammar where verbs are conjugated: e.g. amo, amas, amat. It also covers words which can be declined (in the grammatical sense)... Declined... Do you see where this is going?

The first legionary says 'vae victo, vae victis', which means 'Woe unto the one who has been conquered, woe unto those who have been conquered.' Victo is dative singular, victis is dative plural. In short, he's declining in the grammatical sense.

"So when the second legionary says 'We decline', it brings the whole joke together. Plus there's a natural association between 'decline' and Romans - as anyone who has tried to wade through Gibbon's seminal Decline and Fall of the Roman Empire will know."