Straitjacket is often misrendered as straightjacket.
It's neither intended to make you sit up straight nor to scare you straight, and the wearer's arms are usually twisted up like a pretzel. The 'strait' in 'straitjacket' is the same as in 'dire straits', or, for that matter, the Strait of Gibraltar. Strait is an old-fashioned word for 'narrow' or 'confined', or, in a figurative sense, 'strict' or 'righteous'. The straitjacket was meant to combine both, confining a mental patient for his own good.