A phrase often usd by people who don't really know what they're doing.

This catch phrase was often used by the 80's TV cop Sledge Hammer right before he does something that results in some sort of catastrophic event or disaster.
Captain Trunk: Sledge, you cannot disarm that nuclear bomb!

Sledge Hammer: Trust me, I know what I'm doing.

(A nuclear explosion ensues thus ending the season finale of Sledge Hammer)
by d_art June 29, 2005
what republicans say when voting on a bill, always wrong.
Donald Trump likes to project, "trust me, I know what I'm doing", but that is only in reference to promoting himself.
by Cons are all Santorum February 20, 2016