What does this idiom mean 'to be on a high'?

[Rich7]

Test No. incompl/inter-56 "High and Low", question 1

She's ......... a high at the moment because everything is going her way.

(a) in
(b) on
(c) for
(d) at

Test No. incompl/inter-56 "High and Low", answer 1

She's on a high at the moment because everything is going her way.

Correct answer: (b) on

Your answer was: incorrect
She's for a high at the moment because everything is going her way.
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hi again, I'm back,please explain the hole meaning.

rich7

[Torsten]

She's on a high = she is very happy.
Everything goes her way = everything is like she wants it to be.

TOEFL listening lectures: A lecture from a social science class (2)

[Diverhank]

Is this idiom English English? I've never heard of this before even though I can guess the answer easily enough.

[Mister Micawber]

.
It is familiar to me, and I'm AmE-- in fact, I think we might have originated it, back in the '60s. I'm not sure, though; my memory of those days is a little fragmentary.
.
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Native English teacher at Mister Micawber's

[Diverhank]

Oh OK. Thanks...my circle of friends never used it I guess.