Ok, that manga have now officially thought me something new. I have no idea that "onani" came from a Biblical character. I didn't even know that character existed, much less associating it to a Japanese word.
For the longest time erudite people have snarkily referred to masturbation as "the sin of Onan".
Which is nonsense. Onan's sin was that god directly told him to get his sister in law pregnant, and he was like "nah fam", which god reacted to in his usual extremely reasonable and mature fashion by murdering him dead.