robur

Robur, roboris, n., third declension neuter. The s of the older form robus turned to r between vowels, which is why the stem is robor- and not robos-; the same rhotacism explains honor beside honos. Being neuter, robur serves as both nominative and accusative, so the case has to come from the sentence around it.

It starts as the hard heartwood of the oak and ends up meaning strength of every kind: physical vigour, moral firmness, and the best part of a force - robur exercitus is the backbone of an army, not its timber supply. Robustus is the adjective, and English corroborate is con- plus roborare, to strengthen a claim by propping it up with evidence, which has nothing to do with collaborating with anyone.