A childhood disease of Klingons, roughly equivalent to measles in humans, which caused Worf to collapse on the Enterprise-D bridge and left him deeply embarrassed to have caught it as an adult.
Rop'ngor is an infection that Klingons usually suffer and shake off as children. Catching it as a grown warrior is considered humiliating, rather as an adult human might feel about a nursery ailment.
It appears in the Next Generation episode Up the Long Ladder. Worf faints at his station on the bridge and is taken to sickbay, where Doctor Pulaski quickly identifies the cause. Understanding how mortified he is, she treats him and then tells Captain Picard only that the lieutenant had been observing a Klingon fast and neglected his health.
Worf is touched by the discretion. He thanks her by performing the Klingon tea ceremony in his quarters, and Pulaski, though the tea is poisonous to humans, takes an antidote so that she can share it with him and listen to his recitation of Klingon love poetry.