Are you thinking of "bloody murders" (common in crime fiction or horror) or perhaps "bloody Marys" (the cocktail)?
Is it possible "matures" was intended to be another word, like "bloody matures" for "bloody amateurs" ? This is a very common phrase used to describe someone unskilled. To help me track down the specific text, could you share:
This may be a fragment from a specific conversation, a niche piece of media, or a slight misremembering of a similar phrase. Here are a few possibilities to help you find what you're looking for:
In British slang, "bloody" is often used as an intensive. "Matures" could refer to a specific group (e.g., "Those bloody matures are at it again"), though this is not a standard idiom.
Whether you think it's from a (like British or Australian English)?