You may be aware (or have guessed) that swan, in the kind of context I'm citing, is a euphemism for swear. I think it's mainly used by WASPs* who have grown up in conservative rural settings** where any kind of swearing is frowned upon, so that golly, gee, darn, etc. are the harshest expletives one hears. Indeed, my pious maternal grandmother, Ono, would not say any of these forbidden words.
A former colleague of Steve's used to say "I swan!" -- not something you often hear in an academic library setting, and quite possibly the kind of gaffe that sent him packing after a stint much briefer than he'd anticipated. Around 1977, I dropped this man's teenage daughter at her paternal grandmother's house in Midland, TX, while on my way to visit a friend in San Antonio. I stayed for dinner, where everyone, including her elderly great-grandmother, swanned repeatedly over the girl's growth, sophistication, and recent attainments in Southern California.
In the decade since Alice's passing, I've taken to saying "Well, I swan!" every once in awhile -- usually when I'm with Steve. Sometimes I'll ask him, "Do you swan, too?" and if he's listening, he'll usually say, "I do swan!"
Families should have these verbal rituals. My own mother, Charlotte, upon getting dressed up for an occasion, used to say: "I look better, how do I feel?" BTW, Google gives me no source for this quotation, but I have a feeling it comes from a Mae West movie, and someday I'll take time to ferret it out. Many of Charlotte's favorite locutions (like, "Two to Metuchen," and possibly "Don't pinch the tomatoes!") have cinematic origins. Still living at age 95 at a fairly pleasant level of dementia, she speaks mostly in phrases that come from her long-term memory.***
I had intended to cover a second sense of swan as a verb in this posting, but I've passed the five-paragraph mark. AudioEnglish.net (new to me) gives three senses (news to me!), but I shall take no notice of the third. I mean to dwell on the etymology of the second sense in the forthcoming swan the verb, part 2.
*I do not use WASP (White Anglo-Saxon Protestant) in a pejorative sense. After all, I am one.
**William Faulkner's fictional Snopes family may provide literary precedent for this sense of swan. Something else to look up.
***In Politics and the English Language, George Orwell tells us not to write with phrases that we're used to seeing in print. It occurs to me now that he's warning against a kind of stylistic dementia. More of this idea later.