Sentence:  The letters took their <WSD>source</WSD> from a stream of my imagination in which I was transformed into a young man not unlike my bunkmate Eliot Sands - he of the porch steps anecdotes - who smoked cigarettes , performed the tango , wore fifty dollar suits , and sneaked off into the dark with girls to do unimaginable things with them . 
Meaning of the word marked with <WSD> tags:  the place where something begins, where it springs into being