Meaning
At once; immediately; right away — and also 'all at the same time'. 'Do it one time' means do it right now; 'gimme it one time' means give it to me at once. It can also mean together in one go ('everybody talk one time'). Notably, unlike 'just now' (which means 'in a while'), 'one time' really does mean straight away.
Examples
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Come here one time!
Come here right away!
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Pay it off one time so yuh done wid it.
Pay it off all at once so you're done with it.
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Everybody answer one time.
Everybody answered at the same time.
Origin
From 'one time'; at once/immediately, or all together. See [[just-now]], [[soon-come]].
How it’s used
'At once / immediately' — really means now (unlike 'just now'). Also 'all together'. See [[just-now]].