...what 'pulu si bagoomba' means? (Hint: The Professer told the castaways to shout this phrase to the headhunters so they would leave Gilligan alone.)

It means free the prisoners, It was said in episode #91- "High Man on Totem Pole"

Oh and Professor didn't tell the castaways, he told Gilligan to shout that to free the Howells from the kupaki headhunters, since he looked like the king on the totem pole then later he had to say it for all of them.