na-adjective
seeming good; looking good; appearing favorable
Describes something that appears good, favorable, or promising based on outward signs or impression. Often used in literary or descriptive contexts.
彼の表情は善げに見えた。
His expression seemed good.
その提案は善げに聞こえるが、よく考える必要がある。
That proposal sounds good, but we need to think it over carefully.
良さそう is a more common and colloquial way to say 'seems good,' while 善げ is more literary and less frequently used in everyday speech.
Derived from the classical adjective 善し (よし, 'good') combined with the suffix げ (indicating appearance or seeming). The exact historical development is uncertain, but it follows a common pattern for forming adjectival nouns expressing appearance.