clumsy with one's hands; all thumbs
Describes someone who is not good at delicate manual tasks. Often used self-deprecatingly or to describe a lack of dexterity.
I'm clumsy with my hands and not good at detailed work.
He's all thumbs, so I'm worried about leaving the cooking to him.
A general term for clumsiness or lack of skill, not limited to hands. 手先が不器用 specifically refers to manual dexterity.
Compound of 手先 (fingertips; manual dexterity) + が (subject marker) + 不器用 (clumsy). The expression literally means 'one's fingertips are clumsy'.