Sif

Sif was Thor's favorite wife, and this beautiful giantess was very fond of her flowing golden hair, which represented grain and the harvest. She was very vain about her hair and bragged about it profusely. Meanwhile, on top of that, Thor bragged about her beauty and her faithfulness, and Loki hated it when the gods bragged. It got under his skin and wiggled around like worms. Hearing them brag about how wonderful and perfect they all were made him itch so badly, and the itch wouldn't stop until he scratched it.

In this case, Loki cut off Sif’s hair while she slept, in order to make the itch go away. But really, the question should be asked, did Loki really "sneak in" to cut off her hair like it was told? Because a loss of a woman's long hair in Norse tradition is rumored to have been punishment for infidelity. So... do you think Sif fell asleep next to Loki on purpose and then woke up with her hair missing? Because I do. Bragging about her hair, bragging bout fidelity, and plus with was probably all Loki's revenge for that time that Thor... Never mind. That's a story for a different day.

But Thor was so angry and Sif so distraught, that Thor forced Loki to fix it or Thor would break every bone in Loki's body. An even longer story short, Loki replaced Sif's hair with a golden wig forged by Dwarves, which became even more famous than her original hair.