Hisoka is an interesting fellow. He dresses like he's in the circus, gets excited by strong fighters of all ages, and his Bungee Gum possesses the properties of both rubber and gum. Little is known about his personal life, which makes his quirks all the more perplexing.
Despite how mysterious Hisoka is, there may be a way to discern truths about him through his design. For instance, he seems to have a teardrop design underneath his left eye, something often associated with prisoners in real life. This placement may not be a coincidence; it's possible that Hisoka has served time in jail and had something terrible happen while there. Determining potential implications behind Hisoka's teardrop though will require a deeper understanding of the different meanings behind the tattoo and how they might apply to the clown.
According to dictionary.com, teardrop tattoos are commonly associated with gang prison culture. They often mean that one has served time, been humiliated, or killed someone. They also represent loss or sorrow. Many of these meanings could apply to Hisoka.
Exploring what we know of Hisoka should help narrow down his teardrop's implications. Whether Hisoka has served time in prison is difficult to say for sure, but it's a likely assumption. It's also probable he's killed in his past given how regularly he killed in Hunter x Hunter proper. This, however, raises the question of which of his victims was so impactful that he had to brand himself over it. These two explanations seem like the most likely meanings behind the teardrop, but there may be a little more behind the marking as well.
Hisoka may very well have been humiliated in prison. Since he's always on the lookout for strong opponents, it's possible that he challenged another prisoner who turned out to be a bit too much for him. Something like this actually happens in the Hunter x Hunter manga between him and Chrollo. It's difficult to imagine Hisoka losing, but, if the subject is his younger, less-experienced self, the idea isn't too far-fetched.
In all likelihood, however, Hisoka's teardrop may simply be a part of his shape design motif. Besides the teardrop, he also has a star on his right cheek as well as a heart and a diamond on his outfit. He regularly uses playing cards, which add clovers and spades to his motif. In the manga, all of these shapes can potentially appear at the end of his sentences. If the teardrop has a hidden meaning, then all of these other shapes would have to have meanings, too. Togashi is certainly smart enough to pull this off, but concrete evidence of such significance remains to be seen. For now, Hisoka's teardrop and all his other shapes seem to be mainly aesthetic.
It's also worth noting that Hisoka's teardrop isn't really a tattoo at all. Whenever he's washing himself off, the markings on his face disappear, meaning that they're only makeup. If the teardrop is simply painted on and not inked into Hisoka's skin, then it may not have all that deep a meaning.
There actually is a spinoff one-shot penned by Ishida Sui that goes into Hisoka's past a little. This one-shot, fittingly called Hisoka's Past, shows off a young Hisoka and even shows him applying his makeup for what seems to be the first time. Here, his markings come from circus life rather than prison life. Togashi apparently acknowledges this work, but fans are invited to decide for themselves whether to accept it as canon.
As things stand, the meaning behind Hisoka's teardrop is unknown. It could easily mean that he's been incarcerated or beaten in a fight. It could also just be a part of his look. If Togashi ever decides to explain Hisoka's markings, it at least promises to be a story worth listening to.