でしょう
is basically the [丁]{てい}[寧]{ねい}[語]{ご} of だろう
, which in turn comes from であろう
.
The entry for だろう in デジタル大辞泉 says:
[[連語]{れんご}]《[断定]{だんてい}の[助動詞]{じょどうし}「だ」の[未然形]{みぜんけい}+[推量]{すいりょう}の[助動詞]{じょどうし}「う」》[不確]{ふたし}かな[断定]{だんてい}、あるいは[推定]{すいてい}の[意]{い}を[表]{あらわ}す。
「[彼]{かれ}はきっと[成功]{せいこう}する―◦う」
「むこうの[山]{やま}が[南]{みなみ}アルプス―◦うか」→のだろう
[[補説]{ほせつ}] [現代語]{げんだいご}では、[主]{おも}に「う」「よう」が[話]{はな}し[手]{て}の[意志]{いし}を[表]{あらわ}すのに[対]{たい}し、「だろう」は[広]{ひろ}く[用言]{ようげん}に[接続]{せつぞく}して[推量]{すいりょう}を[表]{あらわ}すのに[用]{もち}いられる。「だろう」を[一語]{いちご}の[助動詞]{じょどうし}とみる[説]{せつ}もある。
This is telling us that だろう
is the irrealis/imperfective form of だ
with the conjecture auxiliary う
, and that it can express conjecture/presumption/hypothesis (uncertainty).
The two example sentences mean (roughly):
- I assume that he will be successful.
- Those mountains over there are Minami-alps, aren't they?
You can see that there is a certain amount of conjecture/presumption going on.
In weather reports, for example, you will hear [明日]{あす}は[雨]{あめ}が[降]{ふ}るでしょう
("it will rain tomorrow"), which is a conjuncture/presumption/hypothesis.
Your question sentence ([嘘]{うそ}でしょ!
) means roughly "you're kidding!", "you don't say!", or "no way!"—one might think of it as an idiom. And, by the way, shortening でしょう
to でしょ
can sound a little feminine.
[多分]{たぶん}
can be used in conjunction with でしょう
, e.g. 多分そうでしょう
("that is probably true"). It just adds a bit more uncertainty to the sentence.
かもしれない
means simply that one cannot say for certain but that there is a possibility.