na-adjective
Describes something done on a large scale, with elaborate preparation or grand design. Often used for events, projects, or schemes.
彼らは大仕掛けなイベントを計画している。
They are planning a large-scale event.
その詐欺は大仕掛けだった。
The fraud was elaborate.
Compound of 大 (large) + 仕掛け (mechanism, setup). The exact historical derivation is uncertain, but the meaning is transparent from its components.