Any of various materials used to dye hair on the head. (noun)
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Use "hair dye" in a sentence
"She is not one of those go-with-the-flow kinds of moms you sometimes see on TV, moms who just sort of roll their eyes and laugh when their kids do some crazy stunt like pour hair dye on the dog’s fur or draw pictures of pterodactyls on the living-room wall with permanent-ink markers."
"Becky Sandler might be especially disturbed to learn of data linking hair dye to a woman’s likelihood of developing lupus, given that she has her roots dyed auburn to cover her premature gray every six weeks."