Strait talking...

Why is it a strait-jacket? Why not a straight-jacket - you know, something to keep you on the straight and narrow?

Pause here whilst you contemplate for a moment.

It turns out that it might be a straitjacket, presumably because you might dash away, or perhaps the hyphen could aid you in escaping from its confines... Which kind of gives away the 'strait' - it comes from 'restricted' or 'narrow' or 'tight', as in the Bering Strait, between Alaska and Siberia, or between the Pacific and Arctic oceans, depending on your point of view...

So is it strait-talking, as in confining your discourse to a narrow (an arrow?) range of subjects, particularly the truth?

Pause here whilst you contemplate for a moment.

Nope, turns out it is straight-talking, as in talking directly to someone and not getting distracted by either homophones or words that sound the same but have different meanings.

And as for strait and narrow, that is obviously a tautology, and so can be rejected strait away.