Why are hordes of screaming barbarians terrifying even to veteran Roman legions?

[Mark Hygate]

.......................... but from experience.....................

And that would be?

I would posit that it has long been accepted that whether it's 'shouting and screaming, chanting and waving of nasty sharp intruments of death, or even organised singing' or 'the silent discipline of massed ranks of identical shields, along with the banging of nasty sharp intruments of death on said shields to a disturbing rythm'; both are done with the equal intent to fire up your own enthusiasm and morale, whilst hoping to intimidate and scare the enemy.

In this particular case, we are variously told that Gauls/Celts are considered both taller and bulkier than the average Roman (moreso indeed in this era when 'Marius' had thrown open recruitment to the under nourished and impoverished) and thus automatically more imposing.

It's also true that we are equally told that the Romans considered the charge of the Celts to be their most dangerous moment.  Much effort being devoted to breaking it up with volleys of javelins followed by classic shield discipline to resist it.  Provided it could be stopped and contained, then the Romans won subsequently - often with massive disparities in casualties.  But if the line were penetrated - all bets were off.

All that preceeds that clash/moment, on either side, is intended to try and affect it - including any screams and warrior-like posturing and otherwise maintaining rigorous discipline.