Hair-Cutting
Unto his lemman Dalida he tolde
That in hise heeres al his strengthe lay,
And falsly to hise fooman she hym solde;
And slepynge in hir barme upon a day
She made to clippe or shere hise heres away,
And made hise foomen al this craft espyn.

The Monk's Tale (ll. 175-180)
in The Canterbury Tales

Most of the illustrations below are from the story of Samson and Delilah (Judges 16). While I could certainly believe that the artists would be depicting Delilah with whatever ad hoc cutting tool a woman might have been handy -- notice their similarity to shears for cutting fabric or shearing sheep -- it seems that these spring scissors may have been a standard implement for cutting one's hair, judging by their presence in scenes in which a novice's hair is cut. Knives appear in a few of these illustrations, too. Pivoted scissors appear more frequently in the 16th and 17th century illustrations.

(For more on the subject of the history of scissors, see Medieval Finds from Excavations in London: Knives and Scabbards or Findings: The Material Culture of Needlework and Sewing.)

Some additional links on medieval & Renaissance hairstyles, including hair care recipes.