What is the difference between ‘wash up’ and ‘wash’?

Question

My clothes were dirty, so I must go and wash.

Can I replace ‘wash’ by ‘wash up’? What is the difference between ‘wash up’ and ‘wash’?

Answer 1

'Wash up' in American English means to clean oneself. It is also used in British English to mean 'clean the dishes after a meal'.

If you use the verb 'to wash' without an object

I must go and wash

it usually means that you will wash your body. There's a less common usage that means to do laundry, as in your example. People usually say which part of the body they are washing.

I must go and wash my hands.
I have to wash my hair tonight.

If you're washing anything other than your body or laundry then the object of the verb is required

I must go and wash the dog
I must go and wash the car

The sentence you offered as an example

My clothes were dirty, so I must go and wash.

has mixed verb tenses and might not be entirely clear to the listener about what you will wash, because the more frequent meaning of 'wash' without an object is to wash the body. It's better to say

My clothes were dirty, so I had to go and wash them.

or

My clothes are dirty, so I must go and wash them.

Answer 2

Please wash up before you eat.

I'm going to wash up before dinner.

Normally used for cleaning hands and perhaps face. Something done somewhat quickly at the sink.

So I went home, washed and dressed, before going to the party.

I scrubbed and soaped and washed till I felt clean again.

Whilst "wash" could also mean cleaning the hands or face ("I'm going to wash before dinner"), it is perhaps used more for more-involved washing, i.e. showers or baths.