Strawheads was a recurrent insulting name given by dark-haired people in Middle-earth to those with lighter locks. It appeared most prominently among the Dunlendings, who gave it to their hated neighbours, the Rohirrim, who they saw as usurping Dunlending lands. Indeed, the only fragment of the Dunlendish language to survive is the word meaning 'Strawheads', Forgoil, seen in the quote above.
The Men of Rohan were not the only 'Strawheads' in Middle-earth. Long before Rohan even existed, during the First Age, the land of Dor-lómin was home to the people of the House of Hador. Like the Rohirrim, these people were noted for their pale hair (indeed, there are suggestions that the Rohirrim may have been descended from them, or at least shared an ancestor). After the Nirnaeth Arnoediad, their lands were overrun by dark-haired Easterlings. It was the Easterling leader, Brodda, who coined the name 'Strawheads' for the newly conquered people of Dor-lómin.