be bored / worried / scared stiff

7. ADV : adj ADV

If you are bored stiff, worried stiff, or scared stiff, you are extremely bored, worried, or scared.[INFORMAL]

• Anna tried to look interested. Actually, she was bored stiff.

Stiff is also an adjective. ADJ : v n ADJ

• Even if he bores you stiff, it is good manners not to let him know it.

.... Cobuild

adj. 11. Excessively high: a stiff price.

adv. 2. To a complete extent; totally: bored stiff.

.... Am-Heritage