A low sound of pain, discomfort, or distress; often used for involuntary vocalizations.
I heard a painful moan from the next room.
He got up with a groan.
Kanji form is uncommon; learners will normally encounter the kana spelling.
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.