A colloquial past-tense conjectural expression meaning 'probably was' or 'I think it was'; used to indicate the speaker's speculation or recollection about the past (forms: だったろう / だったろ).
昨日は雨だったろう。
It probably rained yesterday.
子どものころ、ここに映画館があっただったろう。
I think there used to be a movie theater here when I was a child.
そうだったろ？
Wasn't that so?/Right?
彼の名前は田中だったろうか。
I wonder if his name was Tanaka.