Nibble Heds are a medicinal herb used for regaining one's strength. They are
Nibble Heds
primarily seen in The Little Sisters of Eluria when Sister Jenna gives them to Roland along with her note. She warns that Nibble Heds are only to be taken once an hour otherwise they can cause cramps and even death.
It is possible Nibble Heds could really be "Nibble Heads" of our world, and Sister Jenna just spelled their name wrong (as she also spells 'death' as 'deth'), but there are many Mid-World variations on our own words (such as 'tyger' for 'tiger'), so it is hard to tell how the herb's name is to be spelled.