Having your arms strapped down is not something many of us submit to voluntarily. But when it does happen, is the garment in question a straitjacket or a straightjacket?
Straitjacket picture from Shutterstock
In my customarily dismissive fashion, I've always argued that 'straitjacket' is the correct spelling, reflecting as it does the sense of 'straiten' meaning to restrict. That said, the word 'straight' is much more common, so it's unsurprising that 'straightjacket' also pops up a lot. Indeed, it has now been used so often that the Macquarie recognises that both spellings are acceptable:
straitjacket a kind of coat for confining the arms of violently insane persons, etc. Also, straightjacket.
(Side note: I refer to the Macquarie Dictionary as an authority for Australian usage, but I don't think it's perfect when it comes to defining meaning. The use of 'etc' here adds nothing to the definition.)
Conflicting forces often influence language. You can maintain that 'strait' is the more logical prefix (as I did), but if 'straight' becomes the more accepted spelling, we have to roll with that change. Right now, we're in a transitional phase. As long as you're consistent and don't use both spellings in a single document, you'll be fine.
Final point: whichever spelling you favour, it's a single word. Don't write 'strait jacket' or 'straight jacket'.
Lifehacker's Mind Your Language column offers bossy advice on improving your writing.