Why is 把 in 我去洗把脸 (wǒ qù xǐ bǎ liǎn) = I'm going to wash my face?

Question

YouDao has an example of 我去洗把脸:

知道了，妈。我去洗把脸，让自己清醒一些。
^{Zhīdàole, mā. Wǒ qù xǐ bǎ liǎn, ràng zìjǐ qīngxǐng yīxiē.}
I see, Mummy. I'm going to wash my face and wake myself up.

and I just encountered it in the Chinese TV series 如果蜗牛有爱情, with the English title When a Snail Falls in Love (Wikipedia), in the first episode on QQ about 15 minutes, 49 seconds in:

　

There's similar examples at YouDao for 洗把脸.

It seems weird to me to put 把 in 洗把脸. I would expect 我去洗脸 or 我洗脸去. YouDao indicates 我要去洗脸了 and 把脸洗 are good choices.

Question: Why is 把 in 我去洗把脸?

I see no real reason for 把 to be in there. My guess is that it's a variation on 把脸洗.

Answer 1

把 is used as 量词 (measure word) here.

三、“把”还可以表示动作的次数和量。大部分用于手的动作，如：拉、帮、推、拽、搭、抓等，后面都可以用“把”，用法跟“次”“下”“回”相近。

可是，在现代汉语中，表示动作的量的意义有所扩展，这些动作与手关系不大。

如：表现一把、玩一把、练一把等，时下最流行的一个词就是“过把瘾”。

It's usually used in spoken language, has similar meaning with 次/下. So 洗把脸 is similar to 洗一次脸/洗一下脸, but sounds more vivid and direct.

Answer 2

It is a quantifier, used to count or to express times and frequency.

一洗把脸 means wash my face once. Here the 把 is not a verb but a quantifier. You can view it as an idiom.

Similarly, you can use 把 to express how many times you play(usually video or PC games), such as: I am gonna play a round of PUBG ~ 来一把"吃鸡"（PUBG has a Chinese name called 吃鸡, originated from Winner winner, chicken dinner, so we call it 吃鸡，literally eat chicken (dinner)).

If you ask why do we use 把 to express quantities in this situation, I don't even know, that's why it is idiom, so try play with it, maybe you'll make mistakes, but it's fine.