Half is sometimes used in negative statements, with a positive meaning, to emphasize a particular fact or quality. For example, if you say 'he isn't half lucky', you mean that he is very lucky.
[British, informal, emphasis]
You don't half sound confident.
I didn't half get into trouble.
She eventually decided the acting profession wasn't half bad.
'Presumably this made some impact then.'—'Oh not half.'
See full dictionary entry for half
not half in British English
informal
a.
not in any way
he's not half clever enough
b. British
really; very; indeed
he isn't half stupid
c.
certainly; yes, indeed
See full dictionary entry for half
Examples of 'not half' in a sentence
not half
We are indeed : but not half as irrational as the opposition.
Times, Sunday Times (2016)
The reality was not half as bad as feared.
Times, Sunday Times (2010)
Not half as bad as it could be.
Times, Sunday Times (2013)
It is really not half bad with sugar in it.
E. Nesbit The Treasure Seekers (1899)
Not half bad for a sequel.
The Sun (2014)
For a second choice, not half bad.
Times, Sunday Times (2010)
After all, it is not half so bad as losing an election.
Christopher Hibbert DISRAELI: A Personal History (2004)
But it's not half bad.
Times, Sunday Times (2009)
It's not half as bad as you'd imagine.
Times, Sunday Times (2011)
But, as a second option, it was not half bad.
The Sun (2014)
But then it was over, not half as bad as I had feared.
Times, Sunday Times (2012)
1 (idiom)
not half as clever as he'd thought
Synonyms
not nearly
nothing like
nowhere near
2 (idiom)
Definition
yes, indeed
`Fancy a top up?' `Not half.'
Synonyms
absolutely
yes
`Are you a friend of his?' `Yes'.
indeed
`Did you know him?' `I did indeed.'
ya (South Africa)
`Are you coming now?' `Ya.'
yebo (South Africa, informal)
See half
Additional synonyms
in the sense of indeed
Definition
certainly
`Did you know him?' `I did indeed.'
Synonyms
certainly,
yes,
definitely,
surely,
truly,
absolutely,
undoubtedly,
positively,
decidedly,
without doubt,
undeniably,
without question,
unequivocally,
indisputably,
assuredly,
doubtlessly,
IDD
in the sense of ya
Definition
yes
`Are you coming now?' `Ya.'
Synonyms
yes,
yeah (informal),
sure,
okay,
aye (Scottish, informal),
affirmative (formal),
uh-huh (slang),
yebo (South Africa, informal)
in the sense of yes
Definition
used to express consent, agreement, or approval, or to answer when one is addressed