The Brief: A "sneaky link" is a sexual partner who someone keeps secret.

DEEP DIVE
Someone’s “sneaky link” is a person they’re secretly hooking up with. They may be “sneaky” because they’re having an affair or just because they want to be discreet. Typically these relationships are casual and more sexual than they are romantic. A “sneaky linkup” is when people meet to hookup in secret.
The Ride is a Drake song you won’t feel until it hits you on a cold, late night on the drive back from a sneaky link and all the successes in this world seem attainable.
— 🎹 (@Dr_Sweets23) November 15, 2020
yall pushing 30 talkn bout sneaky link
— l١sh (@alvshaa) November 8, 2020
When my sneaky link tryna “link” 🌚 pic.twitter.com/il3lFFS4Jr
— lover boy bango💔 (@GuhThatsChango) November 16, 2020