Here's why you might have heard it:
* Misheard phrase: It could be a mispronunciation or mishearing of a similar phrase. For example, "let your spirit rock" is a common expression encouraging someone to be enthusiastic or energetic.
* Made-up phrase: It could be a playful or creative phrase someone invented, but it doesn't have a widely understood meaning.
* Specific context: There's a possibility that "let your shamrock" has a specific meaning within a particular community or group.
If you can provide more context about where you heard this phrase, I might be able to help you understand its meaning.