an rip off or a rip off? Which is correct ?唔該!

It should a rip off

an is only used when the word starts with a vowel - a, e, i, o, u
e.g. an apple, an university, an action

a is used for the words that starts with consonants (all letters other than a, e, i, o, u)
e.g. a book, a shirt, a clip, a desk, a rip-off


how rip off is used:
He ripped you off
He is ripping you off
That's a rip-off

a rip-off

rip-off (noun) = with 'hyphen '

rip off (verb)

a rip-off = because singular count noun beginning with a 'non-vowel ' sound, i.e. 不響音

a rip off
解釋 : 偷竊;竊賊

It should be "a rip off" as the first syllable of "rip" is a consonant.