bungler; clumsy person; all thumbs
Refers to a person who is habitually clumsy, awkward, or inept, especially with manual tasks. Often used self-deprecatingly or as mild criticism.
I'm a clumsy person and can't cook well.
He's all thumbs, so we can't entrust him with delicate work.
不器用 is the na-adjective meaning 'clumsy' or 'awkward', while 不器用者 is the noun for a person who is clumsy.
下手 is a general na-adjective for being unskilled at something, whereas 不器用者 specifically implies physical clumsiness or lack of dexterity.
Compound of 不器用 (bukiyō, 'clumsy') + 者 (mono, 'person'). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning.