This saying is in the "One Punch Man" manga. An character named "Speed o' Sound" Sonic uses this as a move against the protagonist of the manga known as Saitama.
This move consists of ten afterimages, proving how fast Speed o' Sound Sonic really is.
Wanker: Because I'm stressed out, I just saw a picture of Jessica Alba, I have an errection, there's an erotic song playing, I'm feeling horny, I haven't had a wank in three days, I haven't got much else to do, no one else is in the house excpet you (why are you still here by the way?!), and I would like to relax and feel wanking is the best option for this
Inquisitive man: So you want to wank for NINE different reasons?
Wanker: Oh and I lost my keys so I want a wank
(Interuption)Conclusive man: You're having a tenfold bash!!
Wanker: Am I? I thought a tenfold bash was wanking ten times in one day?
Conclusive man: No, no. That's defintion one of the same term, the secondary definiton is to wank for ten different reasons, as in Wanker's case today.
Boy1: Alright boy2, do you wan come out to the cinema today?
Boy2: Canβt mate, Iβm on a tenfold bash. I wouldnβt have the time!
Boy1: Come on mate, you can nipp off to the toilets every now and again
Boy2: Nah mate, SERIOULSY a tenfold bash is hard work