Tangled hair. It is said that one of the favourite amusements of Queen Mab is to tie people's hair in knots. When Edgar impersonates a madman, “he elfs all his hair in knots.” (Lear, ii. 3.)
This is that very Mab That plats the manes of horses in the night, And bakes [? cakes] the elf-locks in foul sluttish hairs.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894