noun
very cautious person; very prudent person
A yojijukugo describing someone who is extremely careful, circumspect, and risk-averse in their actions or decisions. Often used with a slightly critical or humorous tone.
彼は慎重居士だから、新しい計画にはなかなか賛成しない。
He's such a cautious person that he rarely agrees to new plans.
あの慎重居士がそんなリスクを取るはずがない。
There's no way that overly prudent person would take such a risk.