also: すけべい
noun, na-adjective
Casual, often humorous or derogatory term for lewdness or a lewd person. Can be used as a noun (a pervert) or as a na-adjective (lewd, perverted).
あの人はスケベな話ばかりする。
That person only talks about lewd things.
彼はスケベだから気をつけて。
He's a pervert, so be careful.
エロい is an i-adjective meaning 'erotic' or 'sexy', focusing on sexual appeal, while スケベ is more about lewdness or perverted behavior and can be a noun for a person.
変態 is a stronger, more clinical term for 'pervert' or 'abnormality', often used seriously, whereas スケベ is lighter and more colloquial.
The origin is uncertain. It may be related to the kanji 助平 (ateji), but the word itself is likely a colloquialism. The katakana spelling スケベ is now standard.