There's plenty of hidden dark spots lurking in Naruto's world - even within the nickname of Fifth Hokage, Tsunade. When Masashi Kishimoto's Naruto story begins, the Hidden Leaf village is led by Hiruzen Sarutobi, the Third Hokage who retook the position following the death of his successor. Sadly, Hiruzen wouldn't last long. The Third Hokage sacrificed himself to repel Orochimaru during the classic Konoha Invasion arc, leaving a vacancy at the top of the village. With few candidates suitable or willing to take Hiruzen's place, Naruto embarks on a quest to bring a famous ninja from the past back to Konoha - one of the Legendary Sannin, Tsunade.
After no small amount of persuasion, Tsunade agrees to take the gig, and while she's certainly a worthy candidate in terms of her shinobi ability, she's not exactly the ethical role model her predecessors were. In fact, Tsunade is notorious for getting drunk and gambling away her money, which is exactly what she's been doing since leaving Konoha some years ago. Such is Tsunade's reputation, her own grandfather (the First Hokage) showed concern when told his little princess was now leading the village. Unfortunately, Tsunade isn't just addicted to gambling, she's incredibly bad at it.
Tsunade's habit for being easily taunted into a bet and then invariably losing her money earned the nickname "the Legendary Sucker" (stop giggling at the back!), as a playful spin on her "Legendary Sannin" title. The moniker began solely due to Tsunade's bad run of betting, as other gamblers saw her as an easy target for a quick win, but the more Naruto reveals about the Fifth Hokage's backstory, the darker her nickname starts to sound. If Tsunade's bad luck stopped at the casino door, that would be one thing, but her uncanny ability to back the wrong horse extends to loved ones also.
Tsunade's younger brother, Nawaki, dreamed of becoming Konoha's Hokage as a child. Tsunade supported this dream and gave him her necklace as a token of support, but Nawaki was tragically killed during the Second Great Shinobi War, not even living into adulthood, let alone surviving long enough to reach Hokage level. As Tsunade fought for the introduction of medical ninja to every three-person team, she found an ally in Dan Katō - a high-ranking ninja who shared her views and wanted to enact that change by becoming Hokage himself. As she did with Nawaki, Tsunade put her faith in Dan, and offered him the same necklace. He died not long after, also during battle. Not only do these tragedies speak to Tsunade's extraordinary run of misfortune, but the "Legendary Sucker" nickname can be interpreted as a reference to her falling for the same dream over and over again - that someone she loves will become Hokage and make everything alright.
Of course, Tsunade's legendary ill luck sets up a meteoric rise for the story's hero, Naruto Uzumaki himself. Though she's not directly related to Naruto, the enthusiastic young ninja shoulders the legacy of Nawaki and Dan (and wears Tsunade's necklace) and fights his way to the top, finally bringing peace to their world of shinobi. Naruto's success breaks Tsunade's run of tragedy, and rewards her for keeping the faith a third time, proving even Naruto's Legendary Sucker can pick right occasionally. Best not take that approach with actual gambling, mind.