noun

• (uncountable, informal) Nonsense; drivel; idle, meaningless talk.
• (informal, in the plural, the blahs) A general or ambiguous feeling of discomfort, dissatisfaction, uneasiness, boredom, mild depression, etc.
• (informal, derogatory) A fool, an idiot.

adj

• (informal) Dull; uninteresting; insipid.
  "Well, the new restaurant seems nice, but their menu is a little blah."
• (informal) Low in spirit or health; down.
  "I decided to go exercise rather than sit around all day feeling blah."

intj

• (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.
  "Yeah, yeah, blah blah blah, Mom, you said this all yesterday."

verb

• (intransitive) To utter idle, meaningless talk.
  "Ooh, I feel so guilty, I've got far too much money — […] So give it away, endow a charitable foundation, burn it, but stop blahing on about it […]"