also: おとっつあん
noun
Affectionate, old-fashioned term for one's father, commonly used until the end of the Meiji period. Now largely replaced by お父さん or パパ.
「おとっつぁん、ただいま」と、昔の子どもは言ったものだ。
Children in the old days would say, 'Dad, I'm home.'
Contraction of お父さん (otōsan) with a colloquial shift; the exact derivation is uncertain.