A meaningless word used by incompetent parents when asked "Why?" by a small child, or a frustrated teenager. An involuntary indication of the lack of intellect behind making unfair, unreasonable, contradictory and arbitrary decisions without any rational analysis, and showing an inferiority complex which refuses to be swayed by logic or precedent (cf. "Because I said so!").
"But I played at Tom's house last week and got home in time for dinner like you said I should, so why not today."
"Anywhy."