na-adjective
seeming lonely; lonely-looking
Describes a person, expression, atmosphere, or scene that appears lonely or forlorn. Often used with 様子, 表情, or 感じ.
彼は寂しげな表情で窓の外を見ていた。
He was looking out the window with a lonely expression.
秋の夕暮れはどこか寂しげだ。
Autumn evenings feel somewhat lonely.
寂しい is the base i-adjective meaning 'lonely' or 'desolate'. 寂しげ adds the suffix げ, meaning 'seeming' or 'appearing', so it describes an outward appearance of loneliness rather than the internal feeling.
Formed from the stem of the i-adjective 寂しい (さびしい, 'lonely') plus the suffix げ (meaning 'seeming' or 'appearance'). The suffix げ attaches to adjective stems to create na-adjectives describing an outward impression.