Could this be some sort of typo

Question

So I'm reading this manga, and at one point person A brings out a syringe and starts explaining to person B about how she needs to recieve what's in the syringe for a procedure to work, and then goes on to start explaining why which is when person B interupts them to say:

えいつ or maybe えいっ

Which is followed by:

Person A: ええ！？> Eeeh?

Person B: どうかしました？ > Is something wrong?

Person A: あはは！ いぇ…あまりにためらいがなかったもので > Ahaha! No...it’s the lack of any hesitation.

I've hit the dictionaries and there isn't anything for えいつ and while there are some entries for えい none of them seem anything relvant.

Given the context to me it seems like B said some equivalent of "Okay", which surprises A who expected to need to talk B into it.

Basically, is this a typo and in either case what's the meaning?

Answer 1

Note that the つ is small, so it's pronounced with the 「えい」sound cut short. As opposed to a えい without a small tsu, where the sound extends naturally.

This is the かけ声 for a sudden, abrupt movement. In this case jabbing with a needle. It carries the feeling of "ためらいがなかった" too, hence this やりとり.

You may also see it for light attacks and is usually used by girls and considered cute. The same spelling may be used for heavier attacks, but will be written with a font, size, etc. to convey a more forceful delivery.