1.
iron plate; steel plate; griddle
A flat piece of iron or steel. In cooking, refers to the flat griddle used for teppanyaki and other grilled dishes.
2.
sure thing; guaranteed; can't-miss
Something that is absolutely certain to succeed or be good. Colloquial slang derived from the idea of something being as solid and reliable as an iron plate.
Usage
Sense 1 is the literal meaning used in cooking and construction. Sense 2 is colloquial slang popular in entertainment and casual speech, meaning something is absolutely reliable or guaranteed to succeed.
COMMON COLLOCATIONS (sense 1):
COMMON PATTERNS (sense 2):
Register
Sense 2 originated in comedy and entertainment circles and has spread to general informal usage. It is not appropriate in formal writing.