Hair  n. 1 a any of the fine threadlike strands growing from the skin of mammals, esp. From the human head. B these collectively (has long hair). 2 thing resembling a hair. 3 elongated cell growing from a plant. 4 very small quantity or extent (also attrib.: hair crack).  get in a person's hair colloq. Annoy a person. Keep one's hair on colloq. Keep calm; not get angry. Let one's hair down colloq. Enjoy oneself by abandoning restraint. Make one's hair stand on end colloq. Horrify one. Not turn a hair remain unmoved or unaffected.  hairless adj. [old english]