Paul Dickson first authored the New Baseball Dictionary in 1999 and it was most recently updated in 2011. "June Swoon" makes an appearance as a popular baseball term and it can be summarized thusly:
"Falling apart after Memorial Day by a team that got off to a good start."
I remember reading this spinoff in my youth: "They are in a swoon in June and they are going to die in July."
Nothing like that popped up in an internet search, and I can recall neither the source nor the team, but I found it very catchy and worth repeating when summers have turned long for our hometown Twins.
Art Rosenbaum, a columnist and sports editor of the San Francisco Chronicle, is given credit for making much use of variations of June Swoon in the late '50s and into the '60s, when the Giants would tear it up in May and fade in June on what seemed an annual basis.
These were the Giants of Mays, McCovey and Marichal, loaded teams that rarely could get through June with a winning record for the month.
Rosenbaum admitted that Bob Stevens, the Chronicle's baseball writer and the 1999 winner of the Hall of Fame's Spink Award, might have been the first to put "swoon" and "June" together after the Giants arrived in San Francisco in 1958.
In Year 1 in the Bay Area and still playing in ancient Seals Stadium, the Giants were 27-17 and tied for first place in the eight-team NL at the end of May, then went 10-17 in June — and the tradition was started.