What does 決まってんだろう imply?


Perhaps specific context is required but what is the real meaning behind this
phrase?




決まる means "to be settled" or "to be decided", and 決まっている means "has been
(already) settled". But as a set phrase, (～に)決まっている also means "it goes
without saying ～", "needless to say" or "certainly (although there is no
direct evidence/guarantee)". See [JLPT N2 Grammar:
に決まっている](https://jlptsensei.com/learn-japanese-
grammar/%E3%81%AB%E6%B1%BA%E3%81%BE%E3%81%A3%E3%81%A6%E3%81%84%E3%82%8B-ni-
kimatte-iru-meaning/)

By extension, when used as the reply to a question, 決まっているだろう/決まっているでしょう
effectively sounds like "Why ask?" or "Of course, ～" or sometimes even "That's
a stupid question". You can think of this as a set phrase to memorize.

決まってんだろう is a contraction of 決まっているだろう:

  * 決まっ **ている** だろう: (literally) "It has been settled, hasn't it?"
  * 決まっ **てる** だろう: ("teiru → teru" contraction described [here](https://japanese.stackexchange.com/a/18159/5010))
  * 決まっ **てん** だろう: ("ru → n" contraction)

