"One-eyed trouser snake" is a euphemism for a penis. The one eye portion refers to the hole where the urine and semen come out. The trouser portion is because normally one has their penis behind a pair of trousers when out in public. The snake refers to the long shaft of the penis, although some snakes are longer than others.
This is considered purple prose if it is used in erotica. In medical terms, the "pee hole" portion of the euphamism is called the urinary meatus. None of these terms sound remotely sexually suggestive, so I would keep away from them when writing erotic fiction. If you're talking to your crude friends, this term is acceptable except in mixed company (crude louts and respectful folks).
Iron Noder 2017