have a notion

[shootingstar]

STAGE MANAGER:
. . .
Over there are some Civil War veterans. Iron flags on their graves . . . New Hampshire boys . . . had a notion that the Union ought to be kept together, though they'd never seen more than fifty miles of it themselves. All they knew was the name, friends - the United States of America. The United States of America. And they went and died about it.
. . .
(Thornton Wilder, Our Town, Act III)

What do you take . . . had a notion . . . to mean in this context?

 

[teechar]

were subscribed to (supportive of) the idea ...

 

[emsr2d2]

believed that ...

 

[SoothingDave]

A "notion" is an idea or belief.

 

[Skrej]

See entry 1a (3)., although to some extent 1a (2)

I personally tend to use it in a sense of a half-formed or incomplete idea, which is more the sense of 1a (2). At least in my region, we often say "I've half a notion to X", which is probably where my sense of it being nebulous comes from. In that context, it also signifies you're not really serious about doing it.