extremely careless; utterly thoughtless
Strongly emphatic, often used in reproach or self-reproach. The suffix 千万 (せんばん) intensifies the base adjective 迂闊 (うかつ).
The project was delayed because of an extremely careless mistake.
How can you be so calm after doing something that thoughtless?
迂闊 means 'careless' or 'thoughtless' without the strong intensification of 迂闊千万.
Compound of 迂闊 (うかつ, 'careless') and the intensifying suffix 千万 (せんばん, 'extremely'). The exact historical derivation is uncertain, but the suffix is used in a few emphatic expressions.