What is the meaning of Blah?

Nonsense; drivel; idle, meaningless talk.

A general or ambiguous feeling of discomfort, dissatisfaction, uneasiness, boredom, mild depression, etc.

A fool, an idiot.

Dull; uninteresting; insipid.

Low in spirit or health; down.

An expression of mild frustration.

(When spoken repeatedly, often three times in succession: blah blah blah!) Imitative of idle, meaningless talk; used sometimes in a slightly derogatory manner to mock or downplay another's words, or to show disinterest in a diatribe, rant, instructions, unsolicited advice, parenting, etc. Also used when recalling and retelling another's words, as a substitute for the portions of the speech deemed irrelevant.

Representing the sound of vomiting.

To utter idle, meaningless talk.

bright

moon

Source: wiktionary.org