I confirmed this with Alan, and he says:
The hiragana is clearly しゅうちんぼん [shuuchinbon], which is the Japanese reading of 袖珍本, meaning a "pocket-sized book" which was perhaps the size of today's paperback books, a handy size for carrying in the sleeves of kimono.
But as to why someone would want to have this written on their swimming trunks, I have no idea. Bizarre!
Of course, we both think it was meant to be a jab at the wearer's testicle size or the popular sport of pocket pool.
P.s. why do people still shop at Wal-Mart?!