noun
A low sound of pain, discomfort, or distress; often used for involuntary vocalizations.
隣の部屋から苦しそうなうめきが聞こえた。
I heard a painful moan from the next room.
彼はうめき声をあげながら起き上がった。
He got up with a groan.
うめく is the verb 'to moan/groan'; うめき is the noun form (the sound itself).
Derived from the verb うめく (to moan/groan). The exact origin of うめく is uncertain, but it is likely a mimetic word related to the sound of groaning.