The formation of 大不了 is a conceptual process where the character 大 ("big") amplifies the notion of 不了, which literally means "not ending" or "not settling". Together, they create an abstract, figurative meaning that visualizes a situation expanding to its maximum possible limit, the "biggest it could get". This imagined extremity is then mentally contained and dismissed, evolving into the colloquial attitude of "at the very worst", conveying a sense of confronting and downplaying a potential adverse outcome.