bù​shìbù​shi

is not

不是 bù​shì frequency
is not adverb
The negative form of the copula verb 'to be', used to negate a noun, pronoun, or identity.

不是学生

bù​shì xué​sheng.

He is not a student.

no adverb
Used as a negative particle to express disagreement, denial, or as a response to a question.

No, you've misunderstood what I meant.

不是 bù​shi
fault adverb
fault; blame

This is all my fault; please forgive me.

Frequency

Movie subtitles
11.2 per million appears in 5% of films
Written text
790.9 per million

Sources

Definitions
CC-CEDICT