The Hair-Thickening Charm (incantation unknown) is a charm which causes the target's hair to grow longer and thicker very quickly. It is presumably used mainly for cosmetic reasons.
Miles Bletchley cast this jinx on Alicia Spinnet from behind while she was doing homework in the Hogwarts library in 1995. His spell, one of several incidents in which Slytherins hexed members of the Gryffindor Quidditch team, caused Alicia's eyebrows to grow at an alarming rate. By the time Alicia reached the hospital wing, they were blocking both her eyes and her mouth. Professor Snape would not accept the testimony of the fourteen eyewitnesses who saw Miles hex Alicia, insisting instead that Alicia must have performed a Hair-Thickening Charm on herself, presumably in a botched attempt to gain a fuller head of hair. Harry Potter may have unknowingly used this spell as a child to re-grow his hair after Petunia Dursley had cut it too short.