Jack's response is not a word in English, nor any language a Terran might recognise. But it's shouted - pitched high and panicked - in exactly the same vein as the word Fuck!

Cutlasses slice above his head in a scissor motion and he dives under the shadow. "Shoot!"

Which is a command, not a censored swearword. He won't be able to Read the bullet, but he'll risk Jo's aim for the kill.