well, I swanee...
I couldn't resist this since I am extremely interested in derivatives.
My Grand Parents used to use I swan and I swanee. The obvious derivative is that both are euphemisms for "I swear". To go further and better explain the swanee vs swan, research shows the terms may be reduced forms of "I s'wan" or "I s'wan ye", northern English dialectal forms of "I shall warrant (you)", more or less equivalent to "I swear".
I know this made your day. :)