The story
From the phonetic similarity to lick my (balls). Modelled after Hellenic terms such as sigma, omega, tagma, etc. as an intentional misdirection.
Etymology from Wiktionary, CC BY-SA 4.0.
ligma
Today it means: A notional disease mentioned to trick the interlocutor into inquiring as to its nature, whereupon the speaker responds by saying ligma balls (lick my balls) as a prank. Full definition →