noun
playboy; playgirl; person with an active sex life
Colloquial and somewhat dated; refers to someone who has many sexual partners or is sexually adventurous. The term is built on 発展 (development) + 家 (person), implying someone who 'develops' romantic or sexual relationships.
See also: 発展 (はってん)
彼は若い頃、かなりの発展家だったらしい。
Apparently he was quite a playboy when he was young.
彼女は発
She's known as a playgirl and always has a new lover.
色事師 refers to a womanizer or seducer, often with a connotation of skill in romance, whereas 発展家 is broader and can apply to any gender.
Compound of 発展 (development, expansion) and 家 (person, specialist). The exact origin is uncertain, but it likely emerged as slang to describe someone who actively 'develops' sexual relationships.