I copy here what I found at Google:
Subject: RE: Origin of 'Do It Again' - Steely Dan
From: Charley Noble - PM
Date: 01 May 03 - 08:29 AM
...the phrase "do it again" does remind me of the old bawdy song chorus which runs:
For a sheepskin she'll do it, she'll do it;
And for a sheepskin she'll do it again,
And for a cow's horn she'll do it at morn,
And merrily turn and do it again.

But I couldn't find it anywhere else, it's a great scottish song, help?