1. Her name literally means 'She Who Brings Victory'
It comes from the old pre-Gaelic word Boudeg meaning 'Bringer of Victory' and hers is the feminine form: Boudega: She Who Brings Victory.
Given how unlikely it is that a mother happens to name her infant daughter for victory, and that daughter happens to grow up to lead one of her nation's greatest revolts, we can safely assume that it was a title. It's the only name we have, though, so we use it.
2. She stood up to the men trying to steal her possessions
We have the barest bones of her story from the Roman historian, Tacitus. He tells us that the king died, leaving half of his estate to the Roman emperor Nero, and half to his wife and daughters, as was normal in a Roman occupied territory.
The Governor at the time decided he should have left it all to Nero, and, in fairness, the emperor was doing his best impression of Donald Trump and had managed to bankrupt the treasury, so this wouldn't have been an unreasonable decision. In this version, Boudica resisted the men who came to take their due and in retaliation, they flogged her and raped her daughters, sparking what is now known as the Boudican revolt.
3. She had already proved herself in battle
This is where we have to read between the lines and fit in what we know. To start with, there is no real need for there to have been a king of the Eceni. In the historical sources at this time there are five name, two are women: the Boudica and the pro-Roman leader of the Brigantes, Cartimandua. So it's entirely possible that our heroine was already a leader. Certainly a hundred thousand warriors were prepared to follow her into battle and, on the whole, warriors follow whoever seems most likely to keep them alive. So my contention is that our kickass woman had already proved herself as a war leader either during the Roman invasion or in the turbulent decades that followed.
4. Hers wasn't the only revolt, but it was the most successful
It certainly seems to have been the best planned. To have been successful, the revolt must have been brewing all winter, with warriors making weapons in their villages and settlements, and when the Governor marched his two legions westward, they were sent the signal to gather.
And so, in the spring of AD61, Boudica led her warriors in the near-annihilation of the Ninth Legion as it marched down from York to subdue her revolt. She went on to burn Colchester, London and St Albans, the three major Roman towns of the time. Given a little more luck she might have destroyed the Fourteenth and Twentieth legions as they marched back from their failed assault on Mona, gone on to assault the besieged Second legion down in Devon and these islands would have been freed of the Roman occupying forces, perhaps forever.
5. She is the only women to have led the combined forces of Britain against an occupying army
And you don't get much more kickass than that!